Kawasaki H1 Technical

Klemm Vintage Kawasaki AHRMA “Sleeper” H1 500

Turning the Widow-Maker into a Reliable, Good Handling, 21st Century Street Bike

A Brief Technical History of the H1 – Kawasaki introduced the H1 500 triple in 1969, and ended production in 1976.  While the H1 certainly evolved in those years, it’s overall reputation as a poor handling motorcycle with a narrow powerband remains even today.  The H1 was not intended to be all things to all motorcycle riders.  Kawasaki wanted to make the first production bike that would be focused almost exclusively at excelling in ¼ mile drag racing…. And not much else.

The engineers who developed the H1 for this ¼ mile excellence, achieved the goals the marketing execs wanted, but they badly missed the mark for most other serious motorcyclists of the day …. And still so today.  To be sure, production H1s had plenty of technical problems that kept them from being a bike for the “serious motorcyclists” of the day.  And as time has marched on, the poor powerband, poor handling, and heavy smoking of the H1 has made it a bike that fewer and fewer vintage owners would choose as their daily rider.

About our H1 500 AHRMA Sleeper Modifications for the H1

The Kawasaki H1 500 continues to be a very popular vintage bike worldwide.  There are no two H1 owners who agree on what modifications make up the “ideal” H1 street bike …. and that’s okay.  For our project H1, we wanted to build a machine that could be a reliable and practical street bike …… that could also win an AHRMA road racing championship.

In the USA, AHRMA is the only truly nationwide race assn, that offers national road racing titles for vintage bikes.  Of the classes that AHRMA permits the H1 to race in, the Heavyweight Historic Production class offered the best opportunity for our “daily-driver/racer” project.  We don’t expect that all H1 owners will want to build exactly the same H1 we have built here, but we do expect that many of the modifications we performed and developed for our bike will be attractive for other H1 owners to apply to their own bikes.  It bears noting that we did everything possible to apply racing modifications that did not compromise street abilities, as well as applying street-ability mods that did not compromise racing performance.

Given that we had to comply with the AHRMA Production Class rules, we were mandated to run stock pipes, stock carb size, stock chassis and rim sizes.  Despite all that, we contend that our finished AHRMA Production Sleeper is very easily as fast and good handling as most “chambers&carbs” cafe bikes.  The difference is that our AHRMA Sleeper offers a level of rider comfort and reliability that few Chambers&Carbs machines can match.  Our Sleeper H1 is not a one-trick-pony specialty bike …. it has the speed, handling, reliability and comfort to be the best of both worlds.

Our H1 500 AHRMA Sleeper performance goals are as follows:

Retain stock pipes, stock carbs, stock engine lower end, and a stock appearance

Deliver reliable street use performance on today’s 91 octane pump gas

Offer reliable 70-75mph cruising speeds, and a peak of 110+mph

Deliver a smooth and linear powerband suitable for street riding

Maintain reasonable fuel range

Reduce street riding exhaust smoke to near non-visible levels

Offer secure high speed handling with a stock chassis

Prove the bike’s speed and handling by winning an AHRMA Production Class championship

Prove the peak speed ability at Bonneville (115mph with a 220lb rider, 4300ft alt)

WE ACHIEVED ALL THESE GOALS   We contend that “all” the technical issues of the H1 are very solvable … and can be resolved without turning the bike into a “chambers & carbs” café racer bike.  More over, our 21st century H1 can look virtually stock to the “vintage-correct” aficionados.

An Overview of our H1 Build-up and Testing

Like many owners, we bought a very “rough” non-running H1 for our project …. We fully planned to do a complete “frame-off” restoration anyway.  The initial build-up of our test bike took 6 months.  We did the initial road testing on a “stock machine” with our 91-octane Head Mod and Decking package.  After completing that, we did street testing of the prototypes for our Stg1, Stg2 and RIV cylinder porting arrangements.

Our candidate H1 was plenty dirty, but it was low miles (chassis) with no signs of ever going down. Our “running when parked” engine had good compression …. but little else was good.  Like many H1s, it had only a few thousand miles on it… but they were all done 1/4 mile at a time. 6 months of cleaning, assembling, and much attention to detail.  The finished test bike started on the first kick, and ran flawlessly.

By August 2011 we were ready to take the H1 to Bonneville.  At Bonneville, our H1 ran consistent passes of 115mph (8100rpm 15/40 gearing) with no tuning or handling issues.  After our final pass, we installed the road-lights in the pits at Bonneville and drained the 32:1 race gas.  We then filled the tank with a 91-octane 60:1 premix, and rode the H1 102 miles to the AHRMA road race event at Miller Motorsports Park, where the bike was displayed in the concourse.

Waiting to be released on to the Bonneville course with the long shadows of morning. Checking spark plugs in our Bonneville pit.  A good view of our air scoop arrangement. Off and away for the 115.594 measured mile run.  220lb rider (fully dressed) at 4300ft.
After our record pass, we re-installed the lights in the Bonneville pits to prepare for the 102 mile road ride to Miller Motor Park. With the Salt Flats in the  background, our H1 cruised comfortably at 70-75mph (5000rpm with our 15/40 Bonneville gearing) Arriving at the paddock area of Miller Motorsports Park with still a bit of fuel left in the tank.

In 2012 we road raced the H1 in the AHRMA Historic Heavyweight Production class at Willow Springs and Miller Motorsports.  We won every race we were in, and were never challenged by any class competitor.

Our rider, Russ Grainger, piloted the H1 to a decisive win on both days of the AHRMA road race at Willow.   The H1 never exhibited a single wiggle, wobble, or any other drama. We ran the license plate for psych effect. Russ put together this You-Tube video of his Willow race.  This footage speaks volumes for the speed and handling of our Sleeper H1.

During the non-racing time in this two year span, the bike was consistently ridden on public highways around the South-western USA.   As of this writing, it continues to be used as a daily rider.

An Overview of the H1s Biggest Problem Areas … and the Solutions.

Building a stock-pipe, stock-carb H1 that runs 110+mph, and handles good enough to win a road race is actually not that challenging.  What is challenging, is making that same 110+mph H1 a comfortable and reliable “around town” and freeway” cruising” bike …. This was our development goal.  The following is a snapshot of the biggest “street-ability” problems we faced …. And an overview of our solutions.  More detailed text of these solutions is on the following pages of this document.

Detonation Related Piston Failures

Detonation Measurement 101 –  For detail on this subject, please read our tech document about 2 stoke Detonation Management.

To summarize the document relevant to the H1, the ONLY way to truly know the detonation risks of a 2stroke is with a modern deto-sensor of the type sold by 2D in Germany.  The introduction of this tool in the mid 1990s was a complete game changer for every 2stroke engine builder worldwide.  It became immediately clear that if you did tuning and development without a deto-sensor, you didn’t have any idea what was “really” happening inside your engine.  It also became clear that all other devices (spark plug temp gauges, EGTs, etc) were suddenly  stone-age tools that gave questionable and erroneous data …. the deto-sensor wasn’t just a good tool …. it was the “only” tool that gave real data in real time.  That is why all the oem manufactures, and all 2stroke Moto GP teams used them.

We started using the deto-sensor in the mid 1990 to develop our endurance racing 3cylinder 2stroke PWCs.  The difference it made in precision engine tuning was night and day.  When we decided to developed our H1 Sleeper, the deto sensor was the primary tool that guided our tuning choices.  It is impossible to overstate how fundamental it was to our testing and development.

The Deto Risks of the H1 –  Even bone stock H1s can occasionally seize or score a piston.  Adding performance mods like chambers, compression, etc, would often compound that piston scoring problem.  There is no single technical problem that causes the many piston seizures that H1s experience, but in general the lion’s share of H1 piston failures were the result of the same source ….. Detonation.  It’s important to note that not all detonation results in piston crown damage or a holed piston.  Milder levels of detonation can super heat the piston, and cause a piston scoring event.  Many piston seizures have been blamed on a cylinder being ‘too lean” in fuel mixture.  In truth, the mixture is often fine, but other design features are causing the increased levels of detonation…. And the super heating of the piston from that detonation was the actual cause of the seizure.  The true solution is not to run “over-rich” jetting, but rather to redesign the parts responsible for the high detonation risk.

In the case of a stock H1 being run on 91-octane fuel, we found that the center cylinder had a consistently higher deto risk than the outside cylinders (on an engine that is free of air-leaks).   It bears noting that premium fuels of the early 70s were 100+ octane with no oxygenates.  To safely run an H1 on today’s 91-octane premium fuels is doable, but it requires a strong accent on anti-detonation modifications, and operation choices that suit the octane you are running.

We offer several engine modification levels for the H1, and all our packages and mods are designed with detonation resistance as a primary feature.  During all of our road testing, road racing, and Bonneville racing, we never once experienced one piston failure or seizure.  We attribute this to the numerous measures we took to reduce detonation risk.

Smoking –  The excessive exhaust smoke of a stock H1 was barely passable back in the day, and downright illegal today.  Many USA states (and various nations) now ticket motorcycles that smoke excessively, and follow that up by disallowing registration renewal until the machine is “fixed”.  In the end, we were able to reduce the smoking of our 21st century H1 to “nearly invisible” levels, during average street and highway riding.  Doing so required the elimination of the oil injection system, and using varying levels of pre-mix for different riding applications…. But it worked great.

Air Leaks –  Engine air leaks of any kind are harmful to the carburetion of an H1 being run at low rpms, and induces a high risk of piston seizure on an H1 being run a higher rpms.  The carb mounting design of the stock H1 has an extremely high risk of allowing air leaks.  As the composite sleeves in the stock carbs age, the risk of them randomly fracturing becomes very high … an so too the risk of a new air leak.  To resolve this problem we convert both the cylinders and stock carbs to allow for a reliable rubber spigot type mounting.  The text below details how this conversion also resolves several other technical problems at the same time.

Vibration –  Early H1s had solid mounted engines, and later (’74-’76) versions had rubber mounted engines.  Regardless of which one you have, every H1 has a pretty serious and uncomfortable high rpm vibration from 5000-6500 rpm.  For racing applications, this is not a big problem because the engine is generally running over 6500 rpm.  However for street ridding, this vibration is a huge problem.  Our solution was to reconfigure the cylinder porting layout to produce a much wider powerband that allowed for freeway cruising and around town riding in the very smooth 3000-5000 rpm range.

Low RPM “Surging” –  When trying to ride a stock H1 at sustained low rpms, the engine often has rpm “surges” even though the throttle position is not being moved.  This surging is not caused by a problem with the carbs or jetting.  The surging is a result of the inlet ports having a much larger volume than what is needed for the engine design.  The stock H1 inlet ports (at the piston skirt) are the area equivalent of a 34mm diameter …. While the carbs are only 28mm.  By reducing the inlet tract volume, the H1 has much cleaner low speed carburetion, as well as the complete elimination of the annoying low-speed surging.   We used two different means of inlet tract volume reduction, and both completely eliminated the surging while having “no” loss of high rpm power.

Poor Handling Characteristics –  The stories of stock H1s doing dangerous high speed wobbles (while turning or simply riding in a straight line) are common and plentiful.  We contend that “any” H1 can be made to handle securely, and run arrow straight at any speed … without any welding on the chassis.  Our 115mph Bonneville passes were made while riding one handed, and during our AHRMA road races, our H1 never exhibited a single wiggle, wobble, or any other evil-handling “antics”…. It was always a safe and solid handling bike.

IN DETAIL  …….. The Technical Obstacles of a Fast & Reliable Stock-Pipe/Stock-Carb H1….And Our Solutions

Head Dome Design and Squish Clearance

Without a doubt, the number one issue that needs attending to for a reliable H1 is the squish clearance and head dome designs.  The shape of the stock head dome has numerous design problems that can quickly induce lethal levels of detonation while running on today’s 91-octane fuel.  As mentioned above, premium fuels were over 100 octane in 1969 when the H1 was developed.  With an octane this high, the risk of detonation was very low … even with a head dome design and squish clearance spec that was far less than ideal,

Re-cutting the stock heads has always been a popular modification for H1s, however just cutting a proper dome design into the heads does not address the excessive squish clearance problem.  To address the squish thickness issue, the cylinders must be decked on both the top and bottom surfaces.  With the proper amount removed from both of these surfaces, our redesigned dome has the ideal squish clearance that offers the biggest possible safety margin from detonation.   With all the added “deto safety margin” gained by this mod package, we have the ability to safely run slightly higher compression ratios than you could run on an engine that has only the heads re-cut…. And we opt for that slightly higher compression to offer our H1 better overall acceleration.

Please note that we do not discuss compression data in PSI numbers because there is such great variation in compression gauges.   The variations are so great the these gauges can only offer general guidance at to whether an engine needs attention.  For more details info about this, please see our document About Squish and Compression Measurement.

As mentioned above, our deto-sensor showed the center cylinder to have a significantly higher deto risk than the outside cylinders. To attend to this problem, we actually cut the center cylinder-head dome to a slightly lower compression ratio.  This “staggered compression” helps greatly to equalize the detonation risk of the three cylinders.

There is a belief that staggering compression puts the engine in some way “out of balance” …. And nothing could be farther from the truth.  In the world of high-bred 3-cylinder two stroke watercrafts and snowmobiles, the manufactures eventually learned that the biggest priority in getting good reliability was making the deto-risk as equal as possible….. Everything else is secondary.  To this end. their high output production 3cylinder models often employed staggered compression ratios, staggered jetting, and even different cdi ignition curves for all three cylinders.  Our many years of building championship endurance-racing 3cylinder PWC engines taught us the exact same truth … and so we employ staggered specifications into this modification, and our other mods for the 500 H1.

If you do no other modification to your 91-octane daily driver H1, our 91-octane Decking and Head Dome mod is the one mod that should be considered as an absolute must have.

The stock H1 dome has an undersized combustion dome, wrong squish angle, and far too much squish clearance (a result of excessive deck height of the H1 cylinders). Our modified head has the dome size and squish angle corrected …… it  yields ideal squish clearances (with the decked cylinders).  The result is a huge reduction in combustion chamber temperatures and detonation risk.  All head and cylinder gasket sealing surfaces are lapped after machining to assure flat surfaces and excellent sealing.

Inlet Manifold Air Leaks

The stock H1 carbs are hard mounted to the solid aluminum inlet manifolds via a slightly collapsible non-metallic composite sleeve.  We have worked with many engines in the past that used this type of mounting…. And we found them to be very problematic.  The biggest problem, is that these inlets are very prone to small air leaks …. And inlet tract air-leaks of any kind can be an engine killer.

We built a fixture that allowed us to test the “air-tightness” of the joint of a stock H1 carb onto the stock manifold …. And the results were dismal.  The stock mounting would produce a completely air-leak free fit about 10% of the time.  There is no doubt that Kawasaki engineers also learned this, and so fit the more reliable “rubber spigot” carb mounts on the 750cc H2 models.

It’s important to note that “ANY” H1 that will be ridden aggressive MUST pass an air-leak test.  For such a test, all 3 inlet ports and exhaust ports are blocked off (usually with expandable rubber freeze-plugs, and then the engine is pressurized to 6-8psi.  The engine should hold that 6-8 psi for 15-20 minutes with “no loss of pressure at all”.  If there is ANY loss of pressure, the leak point must be located and fixed before the engine is run.  For more detail info on pressure testing, read our page on Pressure Testing.

The truth is, having a very small air leak only represents a piston seizure risk on engines that will be run aggressively.   An “easy-use” H1 that never turns beyond 5000rpm is at very low risk of piston seizure from a tiny air-leak…. but an air leaking low-rpm engine is subject to other problems.  Small air leaks like these will cause, difficult starting, inconsistent idling speeds, and random “surging” while cruising at lower rpms.

This photo shows the H1 engine being pressure tested.  The three inlet ports have all been fitted with aluminum plugs in the rubber spigot.  Each plug has an air fitting and hose plumbing that allows all three cylinders to be pressure tested at the same time.  The exhaust ports have been blocked off using expandable automotive rubber freeze plugs.

The engine should be pressurized to 6-9 psi … and it should hold that pressure “with no loss at all” for at least 10 minutes.

In our shop,  we use an air gauge with a regulator to pressurize the engine, however the more simple gauge that comes in our Air Leak Test Kit allows for very easy engine pressuring using a simple bicycle tire pump.

Regardless of the different levels of seizure risks, we consider the stock carb mounting to have an air-leak risk that is absolutely unacceptable for any 500 H1 that will be expected to give reliable operation.  To resolve this problem, we offer a Spigot Conversion modification for the H1 cylinders.  This mod allows for the spigot fitment of any small body spigot carburetor.  For the owners that prefer to maintain as much “stock appearance” as possible, we also offer a spigot mount conversion modification for the stock carbs.  We used these spigot modified cylinders and carbs for all of our road testing, AHRMA road racing, and our Bonneville record runs.   These carbs retain the stock 28mm throat size, and they make jetting changes “A LOT” less time consuming to do.  While the ease of removal, and elimination of air-leak risk were great advantages, this conversion offers another great benefit related to inlet tract volume….. read below for the details.

Inlet Tract Volume

One on-going problem for many H1 owners is the “surging” of the engine during low rpm cruising.  This surging is not caused by carb jetting, but rather by the excessively large volume of the inlet ports that creates very poor “inlet signal”.

Inlet Signal 101  –  Contrary to what some folks may believe, fuel is not pulled from the carb jet circuits by the air that is passing into the carburetor.  In truth, fuel is “drawn” from the jet circuits in the carb by a negative pressure wave that comes up the inlet tract from the crankcase.  This negative pressure wave is referred to as the “inlet signal”.  The volume of the crankcase dictates how much “inlet signal” the engine has to send up the inlet tract … and so this is a fixed value.  It is the engineer’s job to maximize the use of the signal that the crankcase can generate.  Having a strong signal in the inlet tract results in an engine that starts easier, idles better, and responds to carb tuning better……. But sadly, the importance of this concept was not very well understood in the late 60’s when the H1 cylinders were designed.

The stock H1 inlet port window (at the piston skirt) is the area equivalent of a 34mm diameter, but the stock carb is only 28mm.  It is normal for a well designed inlet window to be slightly larger than the area of the carb throat … but not by this big a margin.   The stock H1 inlet port window and inlet-passage-volume are clearly much bigger than they need to be to service the 28mm carbs, and the power needs of the stock-pipe H1.  The result of this excessively large volume inlet passage is “very weak” inlet signal at the carburetor.

The only way to reduce (or eliminate) the H1 surging, is to reduce the volume of the inlet tract.  With this in mind, we manufactured our spigot manifold modification in a way that measurably reduces the volume of the inlet tract.  Reducing this volume increases the strength of the inlet signal …. And eliminates the surging.  The new stronger signal offered by this manifold modification also resulted in much steadier idling, and more responsive differences during carb adjustments.

The stock H1 inlet manifold arrangement.  The total inlet port volume is 47cc. Our (smaller inlet volume) Spigot Conversion. (Inlet passage volume of 41cc) View of the Spigot modified stock 28mm carb body.

Another added benefit of this manifold modification is that it greatly dampens the high-frequency vibration that the carburetors are exposed to.  This reduction in carb vibration also helps the carburetors to meter fuel more precisely at all rpms.

It bears mentioning that the H1 engine generally responds very well (performance wise) to the installation of 34mm carbs because the stock H1 inlet ports are already at that area equivalent.   We stayed with the stock 28mm carbs for our project because stock throat carbs were mandated by the AHRMA Production Class rules for both road racing and Bonneville.  Besides, we did not want to inflict the loss of fuel range that comes with 34mm carbs.  All that said, our H1 blew off plenty of “chambers and carbs” bikes during our road racing …. We thought the 28mm carbs worked out just fine.

Cylinder Porting –   For the H1, we offer three different levels of cylinder porting.  Unlike the porting that has typically been offered for H1s, all of our porting modifications are focused toward getting the best possible low and mid range performance, with peak rpm performance as a second priority.  In truth all our porting levels offer better top end power as well …. it’s just that we don’t focus on that.

The technical background on these port levels is below.  The porting levels that we offer are as follows :

Stage 1 –  This is essentially a blueprinting of the cylinders to eliminate all the port and sleeve mis-matches, as well as perfectly matching the port heights and widths among all three cylinders.  Like all our cylinder porting, the inlet and transfer passages are textured for better fuel atomization, and the exhaust ports are smooth finished to help reduce carbon building up.

Stage 2 – This mod includes all the stage 1 upgrades along with port timing changes to offer a broader power-band.  The big feature of the stage 2 are the piston fed boost ports in the rear of the cylinder.  These ports offer an increase in transfer port area that helps overall acceleration.  We strongly recommend the spigot inlet conversion for the Stage 2 cylinders.

RIV Sleeved Cylinder –  The “Reduced Inlet Volume” cylinders (RIV) offer the very best in broad powerband and good overall acceleration.  We bore out the stock sleeves to replace them with much tougher sleeves made from a high-nickel content alloy.  The RIV sleeves feature a smaller inlet port that offers much better low end power, and big boost ports that greatly increase transfer port area.

Technical Background of our Porting –  Over the last 38+ years, we have developed cylinder porting modification for many dozens of different two stroke machines, and we know generally what kind of power range will be generated by any one set of port timing specifications.  That said, the stock H1 port timings, by most standards, are not very radical.  By all rights, these port timings “should” generate a lot more low end power than the average H1 has.  Of course, it’s not realistic to expect “any” H1 to have stump pulling low end torque.  But that said, stock H1s fall very much short of the low end they “should” have for the port timings they carry …. Why?  We believe there are two basic reasons.  The first is the (previously mentioned) excessively large inlet tract volume.  The second reason is a shortage of transfer port “area”.  We’ve already spoken to the way we deal with the inlet passage volume, but what about the transfer port area?

Sadly, increasing the transfer port area by raising the transfer ports, would result in a narrower powerband…. Not a good option.  The other choice is to “add” additional transfer ports into the cylinder.  Our Stage 2 cylinder porting accomplishes this by the use of adding transfer channels into the cylinder wall (often called “boost ports, or “worm” ports).  These transfer channels are fed fuel mixture through holes drilled in the piston.  While these ports do not move “a great deal” of transfer fuels, they do offer a very effective supplement that makes a very measurable increase in overall acceleration.  An added benefit of these “boost” ports is that they pass cool gases across the underneath of the crown of the piston, thus reducing piston crown temperatures, and also reducing detonation risk.

Our Stage 2 Porting with boost ports that are fed through holes in the piston. The stock oem H1 cylinder porting (47cc inlet port volume).  Installing the Our Spigot Conversion reduces the Inlet port volume to 41cc. Our RIV sleeved and ported cylinder with large boost ports (fed by holes in piston) and 31mm area equivalent inlet port (36.5cc inlet port volume)

Our “RIV” Cylinder Sleeving and Porting –  We believed that the best way to deal with the H1’s porting obstacles, was to install a new steel sleeve that has a considerably smaller inlet port/passage, along with two large and effective transfer “boost” ports in the rear of the cylinder …. This is exactly what we created with our “Reduced Inlet Volume” (RIV) cylinder modification.   The port timings of the RIV cylinder are exactly the same as our Stage 2 porting.  However the changed inlet and transfer port areas of the RIV cylinders yield much better low end performance, along with the same excellent high rpm output.   The inlet port windows of the RIV cylinders are the area equivalent of a 31mm diameter, so the 28mm carbs are still being well served.   The RIV cylinders offer smooth and linear power delivery in the 3000-5000rpm range that is used during inner city and highway driving, as well as stronger mid-range acceleration for acceleration busts.  An added benefit is that the higher nickel-content of the RIV sleeves, which makes them far less prone to wear than the stock Kawasaki sleeve material.  All in all, the RIV cylinder modification is the last word for a stock-pipe, stock-carb H1 that is expected to be street-able “and” race-worthy .

How can the RIV cylinder make good peak power with smaller inlet ports?

The answer is related to “mean-effective port-area.

Mean-Effective Port-Area 101 –  Technically, the term ‘mean effective port area (MEPA)”  refers to the most effective part of a port passageway.  In more plain English,  MEPA is the part of a port that is open the longest.  In the case of a piston port 2stroke like the H1, the MEPA of the inlet port is the bottom 60-70% of the inlet port window because it is open for such a relatively long time (compared to the top of the port).  By contrast, the top 20-30% of the H1 inlet port delivers very little inlet gasses because that part of the port is open for such a relatively short amount of time.  In the case of the stock H1 engine, the stock inlet port has far more port area than what is needed to produce it’s power numbers.   Given that, blocking off the least effective 20-30% of that inlet port area has a negligible impact on the peak output of a stock-pipe, stock-carb setup.

How Does the RIV modification affect Carburetion?

“A Lot”.  The smaller inlet tract of the RIV cylinders has much stronger inlet signal than a stock cylinder, and so it draws considerably more fuel from all the jets circuits with each cycle.… especially the lower speed circuits.  As a result, considerably leaner pilot jets, slides, and needle jets are required for the RIV cylinders.  However once the adjustments are made, the RIV cylinders deliver far better low speed throttle control, as well as consistent, crisp, and clean carburetion at all rpms.

It bears noting that engines with exceptionally strong inlet signal are much less affected by altitude and temperature changes, as well as being more responsive to jetting changes, so the RIV cylinders tune easily.  All this means that with the RIV cylinders, the carbs are far less temperamental.   We did all of our carburetor fine tuning at 400ft altitude.  We then raced the bike at Willow Springs (2500ft), Bonneville (4300ft), and Miller Motorsports (4400ft) with the exact same jetting.  We even tried different settings but none worked better.  This happened because the exceptionally strong inlet signal of the RIV cylinders actually helped to compensate for the air density changes.

Other Inlet Details –  It’s no secret that clamp on “pod” type filters are very popular among the “chambers and carbs” crowd.  We opted not to use pods for several important reasons.  All of our testing and racing was done with the stock “3 into 1” rubber inlet boot.

The stock 3 into 1 rubber boot allows some “pulse sharing” between the cylinders that can be very beneficial to smoother low speed carburetion.  The makers of high performance 3cylinder PWC and snowmobile engines go to great lengths to construct large “common area” inlet boxes to take advantage of this pulse sharing effect.  In addition, the very slight restriction of the 3 to 1 boot actually helps to slightly increase the inlet signal seen by the carbs, and this benefits the precision of overall carburetion… especially low rpm carburetion.  For sure, the individual pod filters are very free breathing, but they offer none of the low speed pulse sharing benefits of the 3 into 1 boot, and do nothing at all to increase signal strength.  Dyno tests showed that pods offered no benefit on our H1.

Our 3 to 1 boot did have some very excessive flash from the molding process that slightly restricted air to some runners, so we simply ground away that excess flash with a small dremel tool.

Because of the slightly change carb angle and inlet length, the 3 into 1 rubber boot will not reach the stock air box.  For air filtration, we fitted with a 4” tall x 4.5” diameter K&N filter to our boot via an aluminum adaptor sleeve. This filter worked great, and made it much easier to move the boot away from the carburetors during carb removal.

The K&N filter with side covers on. A better view of the K&N with the side cover off.

About Pistons –  There are several piston options for building an H1, and there are some important differences

Cast OEM Pistons –  The stock oem Kawasaki cast pistons are very suitable choice for any H1 that will be ridden recreationally.  Sadly, the number of true NOS pistons (and rings) are in very short supply, so they can be an impractical choice for racing or high output applications.  As of this writing, we are not aware of any widely available aftermarket cast pistons for the H1, so we didn’t bother to test with them.

Wiseco Forged Pistons – Wiseco was making pistons for the H1 in the early 70’s.  Sadly, during the 1970’s the materials and design of the 70’s era Wisecos left alot to be desired …. and they were a very poor choice.  In the last 15 years, Wiseco has made huge changes in both their piston and ring alloys.  It addition to this, Wiseco also made big development strides in their cam and taper designs.  All these improvements have made the modern era Wiseco pistons an excellent choice for the H1.  The forged material is very durable, and allows reliable racing performance with piston clearances of .0025.  We strongly recommend the “new” Wiseco pistons, and we strongly discourage the use of the older “blue-box” Wisecos that often become available on Ebay.

Wossner Forged Pistons –  While we did not run the Wossner pistons during our testing, we are familiar with their products, and consider them to be every bit as good as the Wisecos in wear and performance.  There is, however, one importance difference in the Wossner Pistons.  Their crown angle is measurably steeper than the crown angles of the OEM and Wiseco pistons.  We presume this change was made to offer slightly higher compression.  This crown angle difference is very important because when heads are re-cut for our 91-Octane Modification, we cut the head domes to closely fit the pistons that will be run.  If we cut the heads to be run with Wossner pistons, that engine will always need to be fitted with Wossners.   The same would apply to heads that are cut for use with Wiseco/oem crown pistons.  If you later installed Wossners in such a motor, you would have insufficient squish clearance that could result in physical contact between the head dome and piston crown.

About Forged Piston Fables –  There is much chatter about forged pistons needing unusually long warm-up times, needing much greater clearances, and being more prone to seizure …. ALL FALSE.  However there is a very real reason for these false beliefs that is connected to break-in … not piston material.

   The Wiseco piston rings have a thin coating on them that is intended to allow for swifter sealing to the fresh bores….. and this coating must be given some time to literally wear away before the engine is put into hard service.  If the rings are run “too hard too soon” they will expand to a diameter larger than the bore, and “seize” to the bore.  To avoid this scenario, we have a very specific break-in regiment.  We start the engine with fresh pistons & rings, then let it run at a high idle for 60 minutes with a fan blowing on the cylinder & heads.  After this initial run in, we ride the bike, doing no extended high rpm operation for the first tank of gas.  After that break-in process, the Wiseco rings are ready for serious service.   Folks who fail to offer this important run in time will sometimes give the Forged pistons excessive setup clearance to “avert” this failure… but we believe that is doing two wrongs to make a right.  We fit our Wisecos at .0025″ clearance, and have never had a single seizure or scoring event.

Oil Injection and Pre-Mixing –  We hate the term “two-smokers” …. But very sadly, the stock H1 did a fine job of earning and solidifying this term.  Few 2stroke street bikes had a reputation for belching more smoke than the H1.  It’s important to remember that the KMC engineers were saddled with the task of building a machine that was “expected” to have a warrantee…. And also “expected” to deliver 12 second quarter-miles all night long.    Given that, the engineers opted to err on the side of “ample” oil to the engine… with no regard to smoking.  However for our 21st century H1 we needed to be more mindful of exhaust smoke …. And here is the background tech data for the approach we took.

Oil Migration 101 –  Tests were conducted (not by us) to determine the amount of time it took an ounce of oil to “migrate” it’s way into and out of a running two stroke engine.  An oil laced with a radioactive additive was used so the migration could be “seen” and measured with a Geiger counter.  Setting aside all the lengthy boring details of this testing, the summary of the testing was this.  An ounce of oil migrates much faster through a large displacement cylinder than a small displacement cylinder.  In addition, oil migrates through any engine much faster at sustained high rpm, than at sustained low rpm.

To give this information some real world meaning, the engines that need the most oil in the premix would be large displacement singles being run at sustained peak rpm.  The engines that would need the least amount of oil premix would be small displacement cylinders being run at very low rpms.  However, as most of us know, most small displacement cylinders are revved very high, and most large displacement singles are revved very low, so they all tend to use the same range of premixes…. Until you go to the extremes.

We road race Kawasaki 350 Bighorn singles that we hold at 8000+rpm full time.  We race on a 20:1 premix that produces no visible smoke at all while we race.  We experience very little smoking because the oil is “migrating” through our 350cc engines at a very rapid rate.  We ride the same 350cc Bighorns on the street using a 40:1 premix, and on that premix, the bikes never produce “any” visible smoke (but you can smell the bean oil we run).

While many H1 owners prefer to keep their oil injection, we dislike them for a few reasons.  Firstly, despite the variable oil output of the injection system, there is no viable setting that generates acceptably low smoke levels under average street riding conditions.  Secondly, when they fail, there is no warning other than a seized up engine.  Lastly, we consider the oil fitting points to be potential air-leak risk points (if they loosen or break), and that was a risk we did not want to live with.  To gain real control of our H1’s oil diet, we eliminated the oil injection and simply  ran premix.

When we road raced our H1, we started out running 20:1 premix (like our Bighorns), and there was plenty of visible smoke coming from the pipes at all times.  When we put the same H1 on the street with a 40:1 premix, it billowed smoke non-stop…especially during inner city riding.  Why did our H1 billow clouds of smoke on exactly the same 40:1 mix that let our Bighorns run smokeless?? … The answer is Oil Migration.

Mechanically speaking, the H1 is not a 500.  It is actually three 166cc singles.  When we toured our H1 around town at 3000-5000 rpm, the oil migration time was extremely long, and so the oil needed for ample lubrication was very minimal.  For all of our racing we used Maxima 927 Castor oil, and for all of our street riding we ran Maxima Super M for premix (the Super M is arguably the cleanest and least smoky of all premium grade oils).

We began to cut back our oil premix in an effort to get “acceptable” smoke levels during street riding.  This meant no visible smoke while cruising at 3000-5000rpm, and only a minor “blue cloud” while doing “downshift acceleration passing”.  We used a 60 mile loop of varying road conditions to do the evaluation.  After numerous loop rides, these were our results


Sustained 3-5k   inner city riding 7000+rpm  Bursts   in low gears 7000+ rpm  bursts     in high gears Sustained 7000+ running
4oz/3ga   (96:1) Safe –  low smoke Safe –  low smoke Not recommended Not recommended
4oz/2gal  (64:1) Safe but smokey Safe  but Smokey Safe – low smoke Not recommended
4oz/1gal  (32:1) Heavy smoke Heavy smoke Moderate smoke Low smoke


A Few More Notes on Oil and Exhaust Smoke –  We had the privilege to speak by phone with Tony Nicosia about his early experiences with the H1.  Tony was employed, by Kawasaki, to do much of the early testing of the H1s, and he is arguably one of the most successful Kaw triple drag racers of the 70’s.  Tony quickly recalled the exhaust smoke difficulties he encountered.  In the very early going of street testing, he received a ticket for excessive smoke from his H1 prototype test bike.  His solution was to disconnect the oil injection pump cable (holding the pump in “idle” mode full time)….. and then added only one ounce of oil per gallon in premix along with that setting.  Tony claimed that he made “all” of his drag racing passes with this oil setup.  He also made numerous high speed road rides with the same mix …. Including a few peak speed runs from Barstow California to Las Vegas … 150miles (you could get away with that kind of stuff in 1970).  These experiences speak volumes about how little oil the H1 can actually get by on.

We did a few weekend poker runs in our local area where we knew there would be no riding over 60mph, and lots of biker traffic that looked down on smoking 2strokes.  For that, we routinely ran a 102:1 mix (5oz to 4gallons).  The bike emitted virtually no visible smoke during “around town” riding, and only a tiny blue cloud during a few “downshift passing” spurts.  Later teardown inspections showed all the internal parts to be looking very good with no visible “issues”.

These photos are of the left side piston after three very  “spirited” 60 mile loop rides with a 96:1 premix of Maxima Super M and 91-octane fuel.  While most of the riding time was spent at 4000-5000 rpm, there were numerous “passing spurts” beyond 7000rpm. These photos were taken moments after the cylinder was removed.  They show a very visible presence of oil on the piston skirt, and no scoring or scuffing of any kind.  With this mix, we could easily run the recommended .035″ plug gap with no fouling.


Ignition and Timing –  All modern 2stroke 3cylinder (watercraft and snowmobile) engines have cdi ignitions that are programmed with an “advance-retard curve”.   On these ignitions, the timing is very retarded to allow for easy starting.   As rpms increase from idle speeds, the timing advances sharply, to a max advance somewhere between 4000-6000 rpm.  After that mid-range max, the ignition timing begins to retard progressively as rpms increase.  An ignition curve like this offers excellent mid-range acceleration, along with peak rpm performance that has a very low detonation risk.  Unfortunately, no one knew all this in 1971.

Our 1975 H1 test bike had the latest version of the H1 CDI ignition.  This ignition maintains the same ignition timing from 0 rpm, up to 7000rpm.  At 7000rpm, the ignition timing advances very slightly …. This is exactly the opposite of what would be smart.  Why the !!## would KMC engineers do this?…. Simple.  It’s important to remember that the H1 was built as a ¼ mile bike that was expected to be operated at peak rpm in high gear for only a few moments (on 100+ octane pump gas of the day)  After just a few moments at peak rpm in high gear at the end of a drag race, the engine was then quickly shut down.  Under this operational scenario, a small advance above 7000rpm is actually not a bad idea.  Sadly, however, for extended operation above 7000rpm, this added advance a “super bad” idea.  Add in the increased detonation risks afforded by today’s oxygenated 91-octane pump gas, and sustained 7000+ operation is guaranteed to result in catastrophic levels of detonation  …… in less than 30 operating seconds

As the fleet of H1 500s aged, and pump-gas octanes plummeted, the poor head-dome designs and excessive squish clearances of stock H1s would swiftly generate lethal detonation, any time the bike was operated for a brief time over 7000rpm (usually resulting in a seized or holed piston)…. And exactly the same applies to anyone riding an H1 today….. So what is an owner to do?

Octane Specific Riding –  While running a stock H1 CDI ignition, it is impossible to build ANY high-performance H1 engine format that can be operated reliably at sustained 7000+rpm speeds on today’s 91-octane fuel. …. it’s never going to happen.  This applies no matter what pipes or carbs you are running on your H1.  That said, the lion’s share of “stock-pipe stock-carb” riding scenarios CAN be done with 91-octane, or 91-octane mixes.  Here is the background.

During the development of our engine packages, we conducted dyno testing with our real-time detonation sensors hooked up to our H1.  Using this deto-sensor on the dyno gave us a clear insight on the specific detonation risks.  Our H1 showed virtually no signs of detonation at all until the 6000-6500rpm range.  Above 6000 we began to see mild levels of deto on the center cylinder only.  Above 6500, the rate of deto on the center cylinder increased visibly, and the ignition side cylinder began light deto.  As soon as we passed the 7000rpm threshold where the ignition advanced slightly, the center cylinder went into high level deto, with both the left and right cylinders not far behind.  Later tests (with staggered compression and jetting) helped to significantly reduce the deto levels of the center cylinder, however every time we exceeded 7000 rpm there was visible (but now non-lethal) deto taking place on all three cylinders.  These dyno tests were all done running 110 octane race fuel.  The same 7000+rpm passes on 91-octane would have certainly killed a piston.

After these dyno tests, we filled up with 110-octane, and took the bike to Bonneville where we ran 8100 rpm in high gear for the full 50 seconds of our peak speed pass.  A few months later, we took the same setup to an AHRMA road race at Willow Springs Raceway (arguably the highest average speed track in the western USA).  At Willow, the H1 was steadily running 7000+rpms in 4th & 5th gear for 80% of the 6 lap race.  It is impossible to run an H1 harder than we ran this one at Willow Springs.  After each event we removed the heads to check for any visible signs of “destructive levels” of detonation …. There were none.

In between the racing, we ran numerous tests on the street running 91 octane fuel.  As long as we never ran sustained rpms above 6000, the temperatures and detonation were very manageable.  However if we did some aggressive riding that involved repeated 7000+rpm spurts in the higher gears, the 91 octane could no longer run deto safe.  For this kind of riding, we ran a 50/50 mix of race gas and 91-octane to manage the detonation to safe limits.  On a practical note, riding the H1 this aggressively on the street will likely get you thrown in jail in all 50 states…. And we are strongly against racing of any kind on public highways.  That said, everyone has a different definition of “aggressive” riding, so we did this testing to that end only.

For 90% of the street riding that most H1 owners will do, 91-octane in our engine packages will suit most riding applications.  Here is another table to give our experiences more dimension.


Sustained 3-5k inner city riding

7000+rpm  Bursts in low gears

7000+ rpm  bursts in higher gears

Sustained 7000+ running

91 octane



Not recommended

Not recommended

50/50  91 / 110oct




Not recommended

110 octane





Early H1 Ignitions –  Earlier H1s came with points, or the first generation CDI (the one that squeals).  We have not tested the early CDI versions to see if they have the same 7000 advance feature, so we cannot speak to that.  The points ignitions obviously have no advance feature.  However the points ignitions are so wrought with technical problems and weaknesses, we would strongly recommend against building a points equipped machine for sustained high rpm operation.

Other Ignition Notes –  The truth is that there is a modern advance-retard ignition available for the H1 from HPI in Belgium  Horse Power Ignition .  This ignition has a maximum advance at 4000rpm, then retards steadily as rpms increase.  We have not had a chance to test this ignition on our H1, but we feel it could make a huge reduction in high-rpm deto that could very likely allow safe 7000+ operating on 91-octane.  Sadly, this ignition has no charging system for the battery…. That’s okay because the battery can carry a “total loss” stop light for quite a while during daylight riding …. we don’t ride our H1 very often at night anyway.

Temperature Reduction Accessories –  We firmly believe that it’s impossible to build in too much safety margin against detonation.  To this end, we fabricated a simple head scoop to help direct more cooling air to the cylinder-heads of the H1.  Since the scoop is so easy to fabricate from aluminum, we don’t sell them …. but we invite owners to fabricate their own version.

For us the biggest goals of our head scoop was to direct air toward the “forward dome” area of the cylinder head.  In addition, we hoped to direct as much air as possible to the “air-starved” center cylinder.  To that end, we re-located the horn, and used those mount points to attach an aluminum plate that helped to deflect additional air to the center cylinder-head.  Our tests with our deto-sensor showed that the scoop clearly offered a temperature reduction when cruising at highway speeds (and higher).  Sadly, in 2012 AHRMA considered it a “performance enhancing air scoop”…. and disallowed it’s use in road racing.  We still used it during all our street riding.

The very simple 3-peice design of the scoop attached to the heads makes it very easy to fabricate and mount.  Note the center of the top plate (between the frame tubes) is bent upward toward the smaller air guide plate. This view of the scoop shows the top plate located in a way that directs air to the forward side of the dome in the heads.  We felt that this was a very important feature. The simple side panels use a .5″x.75″ alloy bar with threaded 6mm holes to hold the top plate to the sides.  All the plate were made from 1/8″ aluminum.

About Reed Valves –  The AHRMA rules prohibit the addition of reed valves in Production Class … and that was fine with us.  While we did not test reed valves on our H1, we do know that a well executed reed valve conversion design can be an advantage.  That said, We feel that the increased inlet signal, and added transfer area of our RIV cylinders nets a performance result that would be darn close to the performance of a reed conversion (on a machine with stock pipes).  It’s realistic to believe that a reed conversion could yield a much bigger power benefit if chambers are used, but (as mentioned before) we were not interested in developing a “chambers & carbs” package.

Transmission Modifications –  For casual street riding, the stock H1 gearbox works fine.  However for any machine that will be pressed in hard service (road racing or aggressive street riding), the stock transmission gears can be problematic.  When the H1 trans is used aggressively, there is a dramatically increased likelihood of the transmission “jumping out of gear”.  The primary cause of this jumping out of gear is the shifting dogs of the gears being pushed away from each other from the sheer loads of hard acceleration.  To be sure, a bent shift fork can also cause this same problem, but a transmission with perfectly straight shift forks can still jump out of gear.

   To resolve this jumping out of gear, all the gears in the gearbox  need to be “undercut”.  Undercutting is a machine process (typically done on a precision cutter grinder) that cuts an angled surface on the shifting dogs of the gears.  This angled surface helps to actually draw engaged gears toward each other, thus dramatically reducing the occurrence of jumping out of gear.  Undercutting of the transmission gears is a mandatory modification for an H1 that will be ridden aggressively.

  At Klemm Vintage we do the gearbox undercutting, and we offer two different levels of undercutting …. street and track.  In short, the street cut is very effective, but it is a relatively milder angled undercut  that allows for easier and smoother shifting.  The track cut is better suited for high powered H1s that with be run (and shifted) aggressively.  Shifting with the race cut requires a bit more foot shifting force than the street cut, but the gear engagement of the race cut is very “certain”.

We can effectively undercut gears that have experienced occasional jumping out of gear events.  However, repeated jumping out of gear events can damage the gear to a point where undercutting cannot be done.  There is no set specification for evaluating good and bad candidate gears, so all gears are evaluated in our shop on a one by one basis.

Chassis Preparation and Setup


Frame Rigidity and Straightness –  During our initial road testing of our H1, virtually every motorcyclist we encountered told us a horror story about the foul handling and high speed wobbles of “their buddy’s” H1 back in the day.  The only “take-away” we got from all those stories was that any motorcycle can be turned into an evil handling bike if it is assembled and adjusted poorly enough.  For our H1, we approached the handling issues from two separate angles.  The first was to make the existing chassis and components as ridged as possible, and the second was to address the actual suspension hydraulic components.  As mentioned above, this resulted in a bike that handled very predictably and securely at all speeds …. it was a widow-maker no more.  Below are the details of how we prepared our chassis.


About Front End Inspection

Getting good handling from your H1 starts with closely inspecting all your front end parts.  Here is the best approach.

Besides the fork legs being perfectly straight, the surfaces covered by the fork seals should be completely free of any rust or surface dents.  No matter how straight your fork legs “look”, we consider it mandatory to inspect the straightness with an indicator and “V” blocks on a granite surface plate.  Total run-out with the leg supported on each end should not exceed .005”.  If yours are worse than that, they should be straightened by someone with (a lot of) experience doing the job. This is a service that Klemm Vintage offers.

This is the basic setup to inspect the straightness of your H1 fork legs.

If straightening is required, it should be done by someone that has the tooling and experience to do the straightening without leaving any “flat-spots on the leg faces.


Lower Triple Clamp –  The stock lower triple clamp is largely responsible for holding the forks rigid …. And it does.  However the lower clamp is made from relatively soft cast steel that can easily be bent in a moderate “crash event”.  The good news is that a bent lower clamp can be straightened to racing quality.

The clamp can be checked for straightness by fitting the two (perfectly straight) fork legs in each side, tightening the clamps on the legs, and then inspecting the perfect “parallel-ness” of the two legs on a granite surface plate.  If the two legs are not perfectly parallel, the legs should be removed to physically bend the lower clamp until perfectly parallel legs are achieved.   This whole process sounds like a big pain in the butt to do …. But it pays huge dividends in high speed handling stability.  We consider this mandatory for any owner that plans to ride at high speeds..  Klemm Vintage offers fork leg straightening, and lower clamp straightening services for owners who do not have the equipment.

Steering Head Bearings –  The stock “ball-bearing” steering head bearings are adequate for casual low speed riding, but they do not offer the rigidity needed for aggressive street or track riding.  The stock steering head bearings should be replaced with “tapered-roller” bearings (aka Timken bearings).  These tapered roller bearing kits are commonly available from several sources (most popularly from Sudco International).  Installing this bearing kit does require the full removal of the entire fork assembly, but all the front end parts should be removed anyway for inspection of straightness.

Swing Arm Bearings –  The stock H1 swing arm pivots o steel bushings.  Again, these can work fine for an H1 that will be ridden casually.  However for an H1 that will be run aggressively, it is mandatory to replace the steel swing arm bushings with needle bearings.  In addition, there should be radial needle bearings installed on each side of the swing-arm to allow for smooth movement with zero side to side movement (this it common on modern dirt bikes).  There is no purpose made needle bearing kit like this for the H1, but there are some dirt bike kits that can be easily adapted (with some simple machine work).  We opted to start out with a kit offered by Pivotworks made for the 1987 Yamaha YZ125

    This Pivot Works swing-arm needle-bearing kit (PWSAK-Y03-001) contains all the hardware needed for the swing-arm needle bearing conversion.  The two needle bearings press nicely in to each side of the stock H1 swing arm.  However the inner steel sleeves are too long for the H1 application, and must be shortened.  This photo also shows the radial needle-bearings intended to be used on each side of the swing arm.  Cutting the inner pivot sleeve to the “exact” correct length is essential.  If they are cut too long, the swing-arm will have side to side play when the swing-arm bolt is tightened. If the inner sleeves are cut too short, the radial bearings will be bound when the swing arm bolt is tightened.  To be sure it is tedious work, but the end result is well worth the effort.

About Swing Arm Shimming –  The stock H1 frame is often 2-3 mm wider than the total width of the swing-arm assembly.  Most folks simply put the swing arm in position, an use the swing arm bolt to “pull” the frame down on to the narrower swing-arm …. but we didn’t do that. 

  Instead, we added shims on each side of the swing-arm so that the swing-arm assembly was only.2-.4mm narrower than the frame.  By doing so, the chassis is being far less stressed by the tightening of the swing-arm bolt.  It’s a pain in the butt to do this, but it’s small detail that can help handling.

Engine Rubber Mounts –  The early H1s were made with solid mounted engines.  The 73-75 H1s were made with rubber mounting in an effort to reduce the vibration experienced by the rider.  Over time, the stock rubber material of these rubber engine mounts begins to deteriorate to allow excessive engine movement under the loads of aggressive acceleration.

   We have very mixed feelings about how much vibration these rubber mounts really improved rider comfort.  However we had no doubts about the addition movement of the engine in the chassis as the stock rubber material deteriorated.

  Our test bike was a 1975 model with the rubber mounting, and all the rubber cushions were badly deteriorated.   We decided to replace the  stock rubber material with a more dense and stiffer rubber compound.  The material we chose was a Buna-N rubber material from McMaster Car (part# 86975K62).  This 1″ diameter material has a 75A rating on the Durometer scale (the Durometer scale is the international standard scale for rating the hardness of rubber).    The stock  H1 rubber mount material is about 40-50 on this same scale.   We simply removed the stock rubber material from the motor mount sleeves (with a wire wheel), and drilled our new material to fit.  The 1″ diameter Buna-N material drills nicely (in a lathe), and when drilled for a very tight fit on the sleeve, it expands to exactly the correct diameter to fit in to the H1 cases.

Engine Shimming –  Like the swing-arm, the H1 engine is considerably narrower than the bolt mounting-points on the chassis.  Like the swing-arm, we decided to “shim” all our motor mount sleeves so that they were almost exactly the full width of the chassis at every mount point.  Again, this would assure that the chassis would only be stressed minutely when tightening each engine bolt.

  We accomplished this shimming, by adding shim washers between the rubber cushion inner sleeves (that meant the shim washers were at the center-point of the crankcases at each engine mount point).  It bears noting that after we did this shimming, it was clear that our new Buna-N rubber cushions would have to be cut to a “longer than stock” length to securely hold the cases from moving left to right on the rubber sleeve diameters.  The entire process was tedious, and took some time.  However the end result was a rubber mounted engine that fit perfectly into the chassis with minimum “draw-down” stress with the tightening of the motor-mount bolts.

Front Forks –  The AHRMA rules specify that the stock forks must be used… so we were stuck with them.  The stock H1 fork hydraulics and springs are not “ideal” by any standard, but they certainly can be made to work pretty darn good.  The two biggest problems these forks have is, not enough compression dampening for aggressive riding, and too much fork compression when braking heavily.  This excessive fork compression is a big problem because it significantly changes steering angles while entering a turn at high speed.

We originally contacted Race Tech (who specializes in vintage suspension), but they had no kits or mods for our H1 forks.  With that, contacted the folks at Pro Valve in Costa Mesa California.  Pro Valve works with many different kinds of motorcycle suspension, and they felt that they could offer some solutions to our fork issues.

The Pro Valve mod was focused on changing the dampening characteristics with modifications to the dampening rods.  As it turned our, with these mods, the stock fork springs actually worked pretty good..  Pro Valve modified the hydraulic action of the forks to offer much stronger compression dampening in situations where the forks were very suddenly compressed (like braking into a high speed turn).  This added compression damping helped high speed turning stability “allot”.    The modification cost $220, and was well worth the price for a machine that will be ridden aggressively.  The Contact info is Pro-Valve 714 708-2583 ,   nick@provalvemx.com   .

Shock Absorbers –  The stock H1 shocks are 12.5″ center to center, and offer dampening that is far below the needs of aggressive riding.  Going to another shock was a given, however we hoped to address more than one problem when replacing the stock shocks.  Anyone that has ever ridden an H1 aggressively in turns, quickly realizes that the pipes will drag long before the tires start loosing grip.  To help relieve this problem, we fitted a pair of 13.0″ Series-12 Progressive shocks with 1394 springs.  The extra half inch of shock length helped greatly to increase ground clearance of the pipes in high speed turns.  At the same time, the springs on these shocks were clearly stiffer than the stock springs, and so they compressed less during those same high speed turns.  These shocks offered a ride that was “very firm”, and admittedly not very comfortable on rough road surfaces …. but the high speed handling characteristics were excellent.  The only down side to the longer shocks was that we had to increase the size of the clearance dents for the swing arm on the center muffler.

  For our Bonneville record runs, we wanted to lower the profile of our H1 to (hopefully) reduce wind resistance.  For those runs, we fitted a pair of the same 12-Series Progressive shocks, in a 12.0″ center to center length.  Progressive makes no springs for the 12.0″ shocks, so we had to use the lightest spring made fore the 13.0″ units (#1367).  We rode this same set of shocks on our 102 mile road ride from Bonneville to Miller Motorsports.  The ride was a bit firm, but still very comfortable on the smooth highways of Utah.  To be sure, these shocks would not offer good ground clearance of the pipes in high speed turns.  However for general highway cruising and/or drag racing, they would be a very viable choice …. especially for riders with very short inseams.

Wheels and Tires –  The AHRMA mandate that the stock rim sizes and materials be retained.  Given that, we just kept our stock width 18″ rear, and 19″ front rims.  We installed new sealed wheel bearings in both hubs.  We also spent plenty of time getting the stock rims to run as true as possible before mounting tires.  After the tires were mounted and seated, we balanced both wheels using brass bolt on spoke weights that we found on Ebay…..they worked great.

   For our initial testing, and first few road race outings, we ran Avon Road Riders (130/70-18 rear, 100/90-19 front).  These tires offered excellent grip both on and off the track.  We raced the H1 at Chuckwalla Raceway in Southern California (2.5miles 19 turns).  We were easily able to drag the pipes (with 13′ shocks) before we felt any loss of grip.  Since the corner-speed of Willow springs would be much higher, we opted for Racing compound Avons (rear 130/650-18 AM23, front 100/90-19 AM18).  The grip of these non-DOT tires is excellent, and suitable for any road racing application.

Wheel Alignment –  By far, the number one item of importance for secure handling of the H1 is perfect alignment of the wheels.  Over the last 40 years, we have seen and tried many different approaches to alignment, and the one outlined below is the one that offered us the most consistent and precise results.  You’ll need two 8′ lengths of 3″ 90′ angled aluminum extrusion, four equal sized platforms to lay the extrusions on, a wood clamp, and dial calipers.

A)  Remove exhaust pipes, and place bike on box stand that permits alignment rails to pass through.  Place 2×6 wood under front wheel to get a level bike attitude.

B)  Place 8′ rails on top of your platforms on each side of the front and rear wheel

C)  Use a large wood-working clamp to gently hold the rails against both sides of the rear tire.

D)  Use dial calipers to confirm that you have exactly the same distance between the rails at front and rear.

E)  Slowly adjust the rear axle adjusters until the front tire is perfectly centered between the rails at the fore & aft sides of the front tire.

While it doesn’t look very high tech, we have aligned numerous motorcycles in this way, and eliminated the high speed head shake of every single bike. Once we have the perfect alignment, we put a notch in the top flat of both axle adjuster bolts.  Doing so allows us to easily see that we are turning both sides to the exact same rotational spot during future chain adjustments.

As a test, we intentionally mis-adjusted one axle adjuster 1/2 turn from our perfect setting, and then rode the bike. That mis-adjustment induced a noticeable head shake at speeds above 90mph.  We brought the bike back and re-set it to the perfect setting….it then ran 110+ with no head shake at all.

DO NOT believe the adjuster marks on the swing arm… they are NEVER correct.

Klemm Vintage Kawasaki ( source ) Copied so this invaluable information is not lost………………

Chapter 3


The fuel system is composed of the fuel tank and cap, fuel cock or valve, fuel lines, and carburators.  The carburetor air cleaner and its ducting will also be covered here.


The Kawasaki triples have had four different fuel tank caps.  One is a twist-on type with a rubber gasket, which appeared on the H1 and H1A.  It has a vent hole in the center of the inside and outside walls.  There is a baffle in between to prevent fuel loss on acceleration and braking.  If the vent clogs, the fuel will not be able to flow to the carburetors.  To check the vent, blow through it from the outside.


The second type of cap is found on all the 1972 models: H1B, H1C, S2, and H2.  This cap is hinged to the tank at its front edge.  To open, push the cap down, pull the latch up, and release the cap.  The vent is a small hole on the bottom side of the cap outside the rubber gasket by way of a serpentine passage.  Again, to check the vent, blow through the hole.


The third type of cap is similar to the second except it has a locking feature.  To open, unlock the button, hold the cap down, push the button, then release the cap.  This type is used on three 1973 models: the S1A, S2A, and the H2A.

The fourth type of cap is used on the 1973 H1D model, on the 1974 S1B, S3, H1E, and H2B models, and on the 1975 S1C, S3A, H1F, and H2C models.  This type of cap is hinged at the rear.  To open it, hold the cap down, push the latch down, and release the cap.

To clear a clogged fuel tank cap vent on any of the four types, blow it out with compressed air.  The twist-on type of cap is riveted together and cannot be disassembled.  The other three types are easily disassembled for cleaning by removing the three screws on the bottom of the cap.  If the cap leaks fuel when the tank is full, check the rubber gasket for cracks or tears.  To replace the gasket, just pull it over the center section of the cap.

To replace any of the hinged caps or their latches, drive the pin out of the hinge with a 1/16-inch pin punch and a small hammer.  Be very careful not to hit the fuel tank with the hammer.  Hold onto the cap or latch as the pin punch is withdrawn because the cap and latch are spring loaded.  Do not lose the spring.  Start the pin into the tab on the tank before compressing the spring and cap (or latch) into place.  When it is positioned properly, drive the pin in.



Models H1, H1A, H1B, H1C
Lift the seat and remove the single bolt at the rear of the tank. Remove the two bolts on either side of the tank at the front. These bolts also hold on the yellow side reflectors. Turn the fuel cock to the ON or RESERVE position and then pull off all four tubes. NOTE: Fuel will not run from the fuel cock unless it is in the PRIME position. Lift the rear of the tank until the fuel cock clears the frame, then pull the tank straight back.

All Other Models
All other Kawasaki triples have an elastic strap or a “snap-in” type of rubber fitting instead of a bolt at the rear of the tank. The front is held in place by two rubber dampers, one on each side of the frame. The tank has channels that fit over the dampers. To remove this type of tank, lift the seat and remove the strap (if there is one). Turn the fuel cock to OFF in the case of S-series models, or to ON or RESERVE for all others, and then pull off the fuel hoses (three on S-series, four on all others). Lift the rear of the tank until the fuel cock clears the frame, and then pull the tank straight back.

Use a large open-end wrench (27mm for S-series, 30mm for H-series) to loosen the fuel cock ring nut.  CAUTION: Be prepared to drain the fuel into a container.


Check all welded seams of the fuel tank for cracks and signs of leakage, especially around the mounting brackets and fuel valve threaded fitting. Leakage around the filler neck on a twist-on type of tank is usually caused by a deformed flange. To inspect for warping, place a flat surface on the neck flange and look for a gap that would allow leakage.

Drain the fuel tank into a container. Look for paint chips, rust, dirt, or water. Look inside the tank for signs of rust. To clean a rusty fuel tank, pour in kerosene or commercial rust-removing solution and add a number of large bolts and nuts. Shake the tank vigorously while changing its position to scour all the inside surfaces. Empty the tank and flush it with clean gasoline. Repeat the procedure until the tank is cleaned of all loose, scaly rust. If the fuel tank is badly corroded, replace it.

If there is any sign of leakage at the tank’s welded seams, use a commercial epoxy sealant to stop the leak. If the leak is near one of the mounting brackets, it will be necessary to have it brazed. CAUTION: Be careful of an open flame or excessive heat in the vicinity of the fuel tank, as it is potentially explosive because of the vapors.


The S-series Fuel Cock
The S-series machines all use the same manually operated fuel cock, which has three positions. When the lever points straight down, the fuel will flow through the valve if there is more than 1/2 gallon in the tank. When the lever points to the rear, an “S” (meaning STOP) shows on the top side of the lever. No fuel will flow in this position. When the lever points forward, an “R” (for RESERVE) shows on the top side of the lever. In this position, the fuel will flow until the tank is empty. The normal flow position of the lever (straight down) connects the three outlets to a tall, vertical, inlet pipe inside the tank. The fuel will flow through it until the level drops below the top end of the inlet pipe. The RESERVE position of the fuel cock lever connects the three outlets to a short. vertical, inlet pipe inside the tank. Obviously, fuel will still flow into the short pipe when the level is too low for the tall one.

The fuel cock lever turns a barrel valve inside the fuel cock that is sealed in a cylinder of cork. The outer wall of the fuel cock has two holes in its upper side; one to the tall inlet pipe and one to the short pipe. The cylindrical cork seal has two holes that match the two holes just mentioned. The barrel valve is hollow and has two holes through its sides. The holes are not side by side, but are 90° apart. They align with the holes in the cork cylinder one at a time, depending on the position of the lever. The fuel then flows down one of the inlet pipes, through the hole in the cork seal. through the hole in the barrel valve, through the hollow middle, and finally through the three outlets. Of course. each outlet has a tube leading to one of the three carburetors.


Turn the fuel valve lever to the #0 (OFF) position and use a wrench to take off the sediment bowl. Drain the fuel tank by turning the lever to the #2 (RESERVE) position and holding a container under the valve to catch the flow. CAUTION: Don’t allow the gasoline to spill on the hot engine parts, which could start a fire.

After the tank has been drained. slip the fuel lines off their fittings. Use a wrench to loosen the nut joining the fuel valve to the threaded pipe on the fuel tank, and then remove the fuel valve, nut. and gasket from the tank.


Clean the filters around the two inlet tubes, and use compressed air to remove any sediment from the top of the fuel valve. Take off the sediment bowl gasket, retainer, and filter screen. Inspect the screen for obstructions by viewing it against a bright light. Replace the screen if it is torn or cannot be cleaned thoroughly.

Another problem is that the cork seal may have turned in the fuel valve body, restricting the fuel supply channel. To correct this, take out the small setscrew which holds the lever in the fuel valve body, and then pull out the lever. CAUTION: Don’t turn the lever while pulling it out, as this will change the position of the cork seal, if it is loose. Inspect the positions of the holes in the cork seal with respect to the channels in the fuel valve body. If the holes don’t line up, use compressed air to blow the seal out of the fuel valve body. Apply gasoline-proof gasket cement to its outer surface, keeping the glue away from the holes or inner surface. Reinstall the seal carefully, lining up the cork seal holes with the channels in the fuel valve body. Wipe off excess cement before inserting the lever. Let the cement dry, and then soak the fuel valve in gasoline to shrink the cork seal before operating the fuel valve. Install the setscrew into the fuel valve with the lever pointing toward the #1 position. NOTE: Push the lever into the valve while tightening the setscrew, to make sure the screw fits into the lever’s retaining groove.


Position a new gasket inside the joint nut, and then thread it onto the fuel valve by 1/4 turn. NOTE: The joint nut has both right- and left-hand threads. CAUTION: To prevent damaging the threads, install the joint nut on the fuel valve with the collar facing away from the valve. Hold the fuel valve against the fuel tank, and then turn the nut onto the tank’s threaded fitting, which is a right-hand thread. Keep the fuel valve from turning while tightening the joint nut, or else the gasket will be pushed out of its groove in the nut and leakage will result.
Push the fuel line onto the fuel valve fitting, and use a clip to secure it. Position the filter screen, retainer, and gasket up inside the valve, and then install the sediment bowl with a wrench. CAUTION: Don’t overtighten the sediment bowl, or you will tear the gasket. Pour gasoline into the tank and open the fuel valve to prevent drying out of the seal and gaskets.

The H-series Fuel Cock
The H-series machines use a different fuel cock, of an automatic, vacuum-operated type. The lever has three positions. When the lever is straight down, it is in the ON position, and fuel flows from the tall inlet pipe to the three outlets, but only when the engine is running. When the engine stops, the fuel flow stops. When the lever is pointed to the rear, fuel flows from the short inlet, but only when the engine is running. This is the RESERVE position. When the lever is pointed straight up, the fuel cock is in the PRIME position, and fuel flows from the short inlet pipe to the three outlets whether the engine is running or not. The automatic, vacuum-operated fuel cock has a disc-type valve instead of a barrel valve, but the important feature of this fuel cock is the diaphragm-operated needle valve that controls the fuel flow after it has passed the disc valve.

Fuel enters the standpipe and is channeled to the diaphragm valve seat by the lever (in the ON or RESERVE position). When the engine is stopped, this is as far as the gasoline can travel, because the diaphragm and its 0-ring seal are forced against the valve seat by the shutoff spring. When the engine is running, intake port vacuum pulls the diaphragm to the left against the shutoff spring tension, and the 0-ring seal is lifted out of the valve seat. The fuel then passes through the diaphragm valve and fills the sediment bowl. The fuel rises through the filter screen and then flows through the outlet to the fuel line supplying the carburetor float chambers.

It is important to understand the vacuum circuit of the automatic fuel valve in order to service it properly. The right-hand carburetor has a vacuum fitting that is exposed to the vacuum and pressure pulses in the carburetor throat while the engine is running. The vacuum hose transmits these pulses to the fitting on the fuel valve’s diaphragm cover. A check valve inside the cover cancels the pressure pulses, and the vacuum is admitted into the diaphragm chamber. The vent hole in the inner diaphragm cover admits atmospheric pressure to the right side of the diaphragm, and forces it to the left against shutoff spring tension. NOTE: If the vent hole is blocked, the diaphragm will not open properly. When the engine is stopped, the vacuum pulses in the carburetor throat are replaced by stable atmospheric pressure. A small pinhole in the check valve disc admits this pressure into the diaphragm chamber and bleeds off the vacuum acting on the diaphragm. NOTE: If the pinhole becomes obstructed, the diaphragm will remain vacuum locked in the open position. The shutoff spring pushes the diaphragm to the right, and the O-ring seal shuts off the fuel flow through the valve seat.

The automatic fuel valve has a priming mechanism that is used when the carburetor float chambers empty of gasoline, such as after overhauling the carburetors or running out of gas. By turning the lever to the PRIME position, the diaphragm is bypassed and the fuel flows directly through as in an ordinary fuel valve. After starting the engine, turn the lever to the ON or RESERVE position. CAUTION: Don’t park the motorcycle with the lever in the PRIME position, or crankcase flooding may occur.


To check the fuel valve for internal air or fuel leaks, remove the sediment bowl and the vacuum line, then install a spare length of hose on the vacuum fitting. Hold a container under the fuel valve and suck on the hose with the lever in the ON and RESERVE positions. A steady flow of gasoline must come from the fuel valve. and fuel flow must stop as soon as the suction is released. There must not be any vacuum leakage in the diaphragm chamber nor any gasoline coming out of the vacuum hose; either of these problems indicates a defective diaphragm, which must be replaced. NOTE: If gasoline leaks out of the small vent hole in the inner diaphragm cover, the spacer gasket, vent gasket, or diaphragm is damaged.


1) Turn the lever to the ON or RESERVE position, and use a wrench to take off the sediment bowl. Drain the tank into a container by turning the lever to the PRIME position. Slide the fuel lines and vacuum line off the fuel valve fittings. Loosen the joint nut, and then remove the automatic fuel valve, gasket, and nut from the fuel tank threaded fitting. Take out the two screws holding the lever, and then lift off the lever, lever plate, and spring ring.

2) Loosen the five screws on the diaphragm cover a few turns at a time in a crisscross pattern to prevent pinching the diaphragm. Lift off the cover, then carefully separate the diaphragm and spacer from the fuel valve. CAUTION: Don’t pull on the diaphragm if it adheres to the inner cover, or you will tear it. Instead, soak the fuel valve in gasoline to loosen the diaphragm.


Wash the parts in gasoline and then blow them off with low-pressure compressed air. CAUTION: Don’t soak the parts in carburetor-cleaning solvent, which can ruin the diaphragm. Inspect the valve seat surface in the inner cover for nicks or gouging, which can cause fuel flow when the engine is stopped. Suck on the vacuum fitting in the diaphragm cover to make sure the check valve is not stuck; there must be no restriction. Blow into the vacuum fitting to inspect the check-valve disc pinhole; there must be more restriction than when sucking on the fitting, but there must also be a slow air leak into the diaphragm chamber. NOTE: If there is no difference in restriction when blowing or sucking on the fitting, the check valve is stuck. This can cause an erratic fuel supply to the carburetors and poor performance. Use a straightened paper clip to loosen the check valve disc. If the disc pinhole is blocked, use a needle to clear it. Replace the complete fuel valve if the check valve or shutoff valve seat is damaged.

Visually inspect the diaphragm for signs of tearing or puncture. Check the O-ring seal for cuts or gouging, which can cause fuel flow when the engine is stopped. Any damage to the diaphragm or O-ring seal requires replacement of the diaphragm and spacer together. Check the inner diaphragm cover to make certain the vent hole is clear. Inspect the lever gasket for cuts or tears which would cause leakage around the fuel valve lever. NOTE: Damage to the lever gasket, O-ring seal, and diaphragm is most often caused by leaving the fuel tank empty for a long period of disuse, which results in drying out and deterioration of these rubber parts.


3) Wipe off any gasoline from the diaphragm covers and the diaphragm. Position the vent gasket inside the inner diaphragm cover and smear oil around the valve seat surface. Fold up the gasket side of the diaphragm and insert it into the dished side of the spacer. Rotate the spacer inside the diaphragm until the vent holes line up, as indicated by the two matching ears on the spacer and diaphragm.

4) Position the diaphragm on the fuel valve so that the vent holes line up with the vent gasket in the diaphragm cover.

5) Install the shutoff spring over the diaphragm, and then position the diaphragm cover so that the vacuum fitting is in line with the spacer ear. Install the cover screws, with lockwashers, and tighten them in a crisscross pattern a few turns at a time to prevent damaging the diaphragm.

6) If the lever gasket was damaged, install a new one and oil it to prevent damage. Insert the lever, with spring ring, into the lever clamp plate. Install the lever plate with the PRI mark toward the sediment bowl, and turn the lever to the RESERVE position before tightening the two screws. CAUTION: If the lever is installed without the spring ring, gasoline will leak past the lever.

7) Hold the nylon filter by its tab, and then position it inside the fuel valve with the filter holes lined up with the outlets. Install the gasket next to the filter and use a wrench to install the sediment bowl. Roll a new gasket inside the joint nut groove, and thread the nut on the fuel valve by 1/4 turn. CAUTION: To prevent damaging the threads, install the joint nut with the collar toward the fuel valve. Hold the fuel valve against the fuel tank, and turn the nut onto the threaded fitting, which is a right-hand thread. Keep the fuel valve from turning while tightening the joint nut, or else the gasket will be pushed out of its groove, resulting in leakage.

Install the vacuum line, with a clip, on the diaphragm cover fitting. Slide the fuel line onto the fuel valve fittings, and then secure them with a clip. CAUTION: Don’t reverse one of the fuel lines and the vacuum line by mistake, or else the engine will die soon after starting, from fuel starvation. If the lever is turned to the PRIME position, the engine will be flooded through the vacuum fitting on the carburetor. Fill the fuel tank and start the engine to get gasoline into the O-ring and gasket side of the diaphragm and prevent their drying out.




There are three types of air cleaners used on Kawasaki triples. The S-series machines all have a cylindrical paper element, which fits in a can behind the carburetors and under the seat. The carburetors are connected to the air cleaner by three short rubber tubes. The air cleaner has built-in baffling to silence intake noise.

The H1’s all have a conical paper element in the same general location as on the S-series. The air cleaner can is connected to the carburetors by a one-piece rubber molding that incorporates the three tubes to the carburetor mouths and an air plenum chamber below the filter element. Early H1’s had no provision for silencing intake noise, but 1972 models started using a plastic air horn on the H1B. This was not used on the H1C, but appears on the H1D, H1E, and H1F models.

The H2’s all have a conical, oil-wetted foam filter. The location and air ducting are very similar to the H1’s parts. All H2’s have a rubber air horn on top of the air cleaner.

The paper-type air filters on H1’s and the S-series models can be cleaned by blowing dirt off with low pressure compressed air. If they are very dirty, however, they must be replaced.

The H2 air filter is washable in kerosene or a parts cleaner type of solvent. After removing the filter from its wire frame, wash it carefully and dry it completely. Prepare a one-to-one solution of gasoline and SAE 30-weight motor oil. Soak the filter in this solution until it is thoroughly impregnated. Let it dry overnight and the gasoline will evaporate. leaving just the right amount of oil distributed evenly over the foam filter element.

If the upper air filter cap is dirty, it should be brushed lightly. Rub a little SAE 30-weight motor oil into the felt pad to restore its effectiveness.


All Kawasaki triples use Mikuni VM-type carburetors. They are simple in construction but cleverly designed to give a fuel/air ratio well matched to the requirements of the engine under a wide variety of load and speed conditions.

This carburetor has four basic systems: a float system, pilot system, main system, and cold-start system. The float system consists of the fuel bowl, with a float operated fuel inlet needle valve. The fuel flows down from the tank by gravity and into the fuel bowl. As the fuel in the bowl rises to a predetermined level, the float pushes the inlet needle valve closed. The level of the fuel is important to the motorcycle’s performance, as we shall see.

The pilot system is analogous to the idle system on an automobile. Under low-load conditions, it supplies the fuel/air mixture the engine needs. The pilot jet is in a tube leading from the carburetor body or mixing chamber down into the fuel in the bowl. In the front of the carburetor throat is an air inlet leading to a small premixing chamber above the pilot jet. Incoming air is controlled by the air screw. Turning the air screw clockwise cuts down the amount of air admitted, making the pilot mixture richer. Turning it counterclockwise has the opposite effect. In the premixing chamber above the pilot jet is an “emulsion tube,” which is a small-diameter tube with holes. The low pressure produced by the engine sucks the fuel up through the pilot jet and into the emulsion tube. It also sucks air past the air screw and into the premixing chamber around the emulsion tube. The air passes through the holes in the emulsion tube and joins the fuel to make a bubbly froth or emulsion. This mixture is drawn through a short passage and joined by more air from the carburetor throat that comes in through the bypass hole. This final mixture flows into the carburetor throat through the pilot outlet hole and goes into the engine. If the throttle is lifted a little bit, the flow in the bypass reverses and the fuel/air emulsion flows out through both the pilot outlet and the bypass.

The main system comes into play from idle speed to full throttle. All the fuel for the main system comes through the main jet. Like the pilot jet, this one is located in a tube extending from the body of the carburetor down into the fuel bowl. Of course it is much larger than the pilot jet. and protrudes from the end of the tube into the very bottom of the bowl. The main jet feeds fuel to the needle jet. Down the middle of the needle jet and partially blocking it is a tapered rod called the jet needle. This is carried by the throttle slide, a pistonlike valve that rides up and down in a bore in the carburetor body to control the amount of air going into the engine. As the slide is raised and lowered. the taper of the jet needle changes the amount of blockage of the needle jet. This varies the amount of fuel which can get through the needle jet and to the engine. At low speeds, the slide closes the venturi, restricting the amount of air available to the engine. At the same time, the larger diameter upper end of the jet needle blocks the outlet of the needle jet, allowing less fuel to flow. At large throttle openings, the slide opens the venturi and a lot of air can come through. Because the slide is so high, only the sharp tip of the jet needle hangs down into the needle jet. resulting in a greater fuel flow. When the jet needle is pulled so far out of the needle jet that the needle jet’s flow capacity exceeds that of the main jet, the total fuel flow is determined by the main jet alone. This happens over about 3/4 throttle.

Another feature of the main system is the primary choke of the needle jet. The primary choke is a little wall from 2 to 8mm high on the front of the needle jet opening, extending tip into the carburetors venturi. The primary choke acts in conjunction with a tiny fuel reservoir around the top of the needle jet and an air passage to the reservoir from the mouth of the carburetor. Together they act to keep the fuel mixture lean at low power outputs to help the engine run more smoothly. At high throttle openings, they enrich the mixture to protect the engine from overheating and possible seizure. The higher the primary choke is. the greater the combined effect.

The fourth system of the Mikuni VM carburetor is the cold-start mechanism. This is a special system that takes the place of the choke on an automobile carburetor. When activated, it supplies an extrarich mixture to the cold engine for starting. An emulsion tube extends into a well in the fuel bowl. At the bottom of the well is a small, fixed, brass cold-start let. A large air passage leads from the mouth of the carburetor to a mixing chamber on the side of the carburetor body at the rear. A large, flat-fronted valve in the chamber closes a fuel passage that conies up from the emulsion tube. To start a cold engine, the cold-start valve is lifted with the throttle closed. The engine sucks fuel through the cold-start jet, up the emulsion tube (air is supplied by the air space above the fuel in the bowl), and into the mixing chamber. There it is joined by more air, and the mixture is drawn down a passage which empties into the carburetor throat downstream from the throttle slide. The throttle slide must be closed for the cold-start system to be effective.


See the engine disassembly section of Chapter 4, Engine Service, for carburetor removal. Because of the possibility that gasoline will spill out of the carburetors while you are disassembling them, work on a surface that will not be harmed by it. CAUTION: Fire danger is extreme. Do not smoke while working on the carburetors or work near an open flame until the carburetors have been completely disassembled and dried.


1) Unscrew the ring nut, then lift the throttle slide assembly out of the carburetor body.

2) Push together the ring nut (with the carburetor cap in it) and the throttle slide assembly to compress the slide spring. This will cause the throttle stop rod to protrude from the idle speed adjuster. Remove the cotter pin from the end of the throttle stop rod. The throttle stop rod will fall out of the bottom of the slide.

3) Push the throttle cable sheath down into the adjuster while holding the throttle slide and the carburetor cap aligned as shown. Disengage the cable nipple from the keyhole-shaped hole in the center of the throttle slide. There may be a needle retainer in the slide under the spring, in which case the spring must be compressed separately and pulled completely out of the slide to allow the retainer to move far enough to let the cable nipple move to the large end of the keyhole-shaped hole in the slide.

4) Pull out the needle retainer, then push the needle out from the bottom. CAUTION: Do not remove the small E clip on the top end of the needle. If the E-clip were accidentally moved to a higher notch, the carburetor would supply a leaner mixture, which would cause overheating, detonation, and engine seizure.

5) Use a 12mm wrench to remove the cold-start cable adjuster from the carburetor body. Compress the spring enough to disengage the cable nipple from the plunger. The spring and adjuster will now slip off the end of the cable.

6) Unscrew the air screw, and remove it and the spring from the carburetor body.

7) Remove the four float bowl screws. If there is an overflow tube bracket held on by one of the screws, note on which corner of the bowl it goes to speed assembly. Lift off the bowl and remove the gasket between it and the carburetor body.

8) Slip out the float pivot pin, then remove the float. Use a small flat-bladed screwdriver to remove the pilot jet. Use as large a flat-bladed screwdriver as possible to remove the main jet. CAUTION: These jets are brass and rather soft. Be careful not to let the screwdriver ruin the slot. Do not push wires or small drills through the jets. Turn the carburetor body right side up and catch the
float valve needle as it falls out. Remove the float valve seat with a 10mm socket wrench.

9) Lift the needle jet out of the carburetor body from the top. It may be necessary to tap it from the bottom with main jet removed. Do not damage threads!


Soak all carburetor parts, except those made of rubber, in solvent or a commercial carburetor-cleaning solvent. Rinse the parts thoroughly in hot water to remove the solvent. Use compressed air to blow out all jets and passageways. Inspect all jets and passageways for deposits caused by stagnant gasoline.

Check the float valve needle and seat for pits or grooves. Submerge the float assembly, and then shake it to listen for gasoline, which would indicate a leak. NOTE: A leaking float causes high fuel levels, with consequent rich fuel-air mixtures and flooding. Dark scratches on the brass floats indicate contact with the carburetor body caused by an incorrect float level adjustment. Check the floats for a convex shape, which is normal. Concave floats have been collapsed by using compressed air on an assembled carburetor, and they will result in excessively rich fuel-air mixtures. Insert the float hinge pin in the carburetor body and check for a snug fit.

Compare the markings on the main jet, jet block, jet needle, low-speed jet, and throttle valve cutaway against specifications.



Inspect the throttle slide for wear on its outer surface. If the plating has worn through, replace the throttle slide. NOTE: A worn throttle slide is evidenced by a clicking sound at low throttle openings. Insert the throttle slide in the carburetor body and check for free movement. If binding is evident, replace the carburetor. CAUTION: A sticking throttle slide can cause loss of control from a runaway engine.

Roll the jet needle on a flat surface to check for bending, and inspect the tapered section for nicks or wear. Make sure the clip is tight in the jet needle groove and the retainer is not bent or broken. NOTE: A loose jet needle flutters in the jet block and causes erratic engine operation at part-throttle openings.

Check the rubber seal in the end of the cold-start plunger for cracking or deterioration. Insert the plunger into the carburetor body and check for free movement without excessive play. To inspect the plunger for leaking in the off position, install the plunger, spring, and nut in the carburetor body. Wrap tape around the pickup tube, and then blow into the tube. There must not be any leakage past the plunger seal, which would cause flooding at low throttle openings. If leakage is evident, check the plunger bore in the carburetor body for damage and inspect the plunger seat for nicks.

To check the carburetor fuel channel, hold a finger over the float valve hole, then blow into the fuel line fitting. Leakage is caused by a porous carburetor casting; therefore, you must replace the carburetor.

The fiber insulating sleeve of a spigot clamp-type carburetor must not be worn or cracked. NOTE: On H2 models, check the rubber socket on the cylinder intake flange for poor bonding to the metal flange, which can result in an air leak. CAUTION: A leaking carburetor-to-manifold connection results in excessively lean air-fuel mixtures, with consequent piston seizure and engine overheating.

Inspect the throttle and cold-start cables for fraying or corrosion. Make sure the cable action is free of binding. Make sure the throttle stop rods are straight by rolling them on a flat surface. CAUTION: A bent or nicked throttle stop rod can cause the carburetor to stick at wide-open throttle.


1) Drop the needle jet into the carburetor body from the top. The notch in the side of the jet fits onto a pin in the body near the bottom, as shown. NOTE: The pin must be tight in the carburetor body.

2) Turn the body over. Put the washer on the bottom end of the needle jet, and then thread the main jet into the bottom of the needle jet. CAUTION: Be sure the main jet is a reverse-type, round-headed jet. A hex-head main jet has different-sized threads and can strip the threads in the needle jet. Check the size in the specification table in the Appendix. Too small a main jet can cause major engine damage from overheating. Too large a main jet will cause excessive exhaust smoking, high gasoline consumption, excessive emissions, and poor high-speed running.


3) Tighten the main jet to 17 lb-ins. CAUTION: Be sure the tip of the screwdriver fits the slot in the jet. The main jet is soft brass and can be damaged easily by using too narrow a blade or by overtightening. After tightening the main jet, look into the air jet passage in the mouth of the carburetor to check the alignment of the needle jet air hole. CAUTION: If the air hole is blocked or masked, midrange and high-speed operation will suffer unless the needle jet is replaced.

4) Check that the fuel inlet passage is clear, and then install the needle valve seat with its washer. Tighten the seat securely. CAUTION: Do not overtighten or the carburetor body will be damaged.

5) Drop the needle valve into the seat with the sharp end down. Suck on the fuel line while holding the valve against the seat, to check for leaks that will cause flooding or fuel overflow. Leaks can occur at the needle and seat and through casting flaws in the carburetor body.

6) Position the float assembly as shown, and then slide the pivot pin into place. Note that the soldered tangs on the floats point toward the main jet. CAUTION: Be sure the floats have not been crushed or bent so as to interfere with the carburetor body or float bowl. The pivot pin must be a fairly snug fit in the carburetor body posts or the float valve’s performance will be erratic. Hold the carburetor right side up and check that the float does not drop far enough for the needle valve to fall out of its seat. CAUTION: If the needle valve falls out in operation, the carburetor will immediately overflow, flooding the engine inside and out, which is a dangerous fire hazard. The float on early models cannot be adjusted and must be changed for one with less drop. Later models have an adjusting tab. which can be bent to obtain a maximum float drop of 20mm.

7) To measure the float level, rest the carburetor body on a horizontal flat surface, with its air intake pointing straight up. Tip it back until the float arm just touches the valve tip. Measure the distance from the float bowl gasket surface (without a gasket) to the outermost edge of the float. which must be as specified in the Appendix. If it is incorrect, bend the tab that bears on the end of the needle valve. NOTE: Too high a float level will cause major engine damage from overheating. Too low a float level will cause excessive exhaust smoking, high gasoline consumption, excessive emissions, and poor highspeed running.

8) Drop the pilot jet into the tube behind the main jet, with the bleed holes down. Check that the pilot air and fuel passages in the carburetor body are open. CAUTION: Clogged pilot system passages can cause poor low-speed running. If they are blocked completely, the engine will not idle at all. The bleed holes in the jet must also he free of debris. CAUTION: When replacing pilot jets, be sure they have ISO threads. This is commonly indicated by a punch mark on the face of the jet near the screwdriver slot. If a jet with incorrect threads is used, the carburetor body will be damaged. See the beginning of Chapter 4, Engine Service, for a complete explanation of ISO threads. Carefully tighten the pilot jet with a small screwdriver.

9) Install a new gasket so that the eyelet on the gasket fits over the tube for the cold-start jet. Check the inside of the float bowl to be sure the brass cold-start jet is in the bottom near the starter reservoir tube. The brass overflow tube must be tight in the bowl. CAUTION: If the overflow tube loosens or falls out during operation, gasoline will run out of the bowl, causing a fire hazard. Check that the air vent in the carburetor body is clear. H1 carburetors have an extra vent that opens into the mouth of the carburetor, which must also be open. CAUTION: If all the air vents are stopped, the float chamber will be under pressure when the fuel cock on the tank is opened. This will force the gasoline through the needle jet and into the venturi of the carburetor, creating a fire hazard or a severely flooded engine. Position the float bowl on the gasket.

10) Fasten the float bowl in place with four screws, each with a lockwasher. Don’t forget the overflow tube holder on H2 and S-series carburetors, which goes on one of the screws closest to the engine. Tighten the screws evenly to prevent distorting the fuel bowl.

11) Screw in the air screw with its tension spring. CAUTION: Do not tighten this screw excessively; bottom it lightly to prevent damaging the seat in the carburetor body, which would make an idle mixture adjustment difficult. This can only be cured by replacing the carburetor assembly, or the body, if one is available.

12) Push the rubber dust cover over the end of the starter cable, and then slip the adjuster on as shown. Put on the return spring, and then slip the valve plunger over the cable nipple.

13) Screw the adjuster into the carburetor body and tighten it carefully.

14) Drop the jet needle. with its clip installed, into the center of the throttle valve slide. See the specification table at the end of this chapter for the proper clip position. Push the retainer down on top of the jet needle so that the retainer does not cover the other holes in the slide.

15) Insert the slide-return spring into the slide and push the throttle cable through the ring nut and through the adjuster in the carburetor top. Position the carburetor top against the return spring. Now push the cable nipple through the double hole in the slide. NOTE: Be sure the gasket is in place. The cable nipple will hook into the other side of the double hole when released. Now use a pointed instrument to rotate the needle retainer until it is in a position to prevent the cable from slipping back into the large side of the double hole. NOTE: Some models use a retainer with a small tab that fits into the double hole to prevent the cable from slipping out.

16) On H1 and S1 models only, slip the throttle stop rod up through the last hole in the throttle valve slide. The rod should extend through the slide, inside the spring, and through the idle adjuster screw on the carburetor top as shown. Insert a small cotter pin in the hole in the end of the rod to secure it.

17) Install the completed slide assembly into the throttle bore of the carburetor body. The groove in the slide fits on the pin in the side of the bore to prevent the slide from rotating. The needle goes into the needle jet. The key in the carburetor top fits the notch in the body, as shown in Step 15). Screw the ring nut on finger tight. CAUTION: Be sure the gasket stays in position. If the gasket slips out of position, it could prevent the throttle slide from moving freely. If it loosens during engine operation, it can be tightened by tapping lightly on the ridges with a screwdriver and a mallet.

18) Install the side throttle-stop screw, with its tension spring, on H2’s, S2’s, and S3’s. Screw it in until it just lifts the slide. CAUTION: If it is screwed in all the way, the engine will race when started.

19) Even though the float level has been set on the bench. this does not guarantee that the fuel level will be correct. It is the fuel level that actually determines how rich the mixture will be. To measure the fuel level, drill a small hole in the bottom of an old float bowl and thread it to accept a small brass fitting, onto which a neoprene tube should be fitted as shown. Though the illustration shows the carburetor mounted on a test stand. the level can be similarly checked on a motorcycle. Find the fuel level for your machine in the specification table at the end of the chapter. That level is the distance in millimeters from the center of the carburetor bore to the level of the fuel in the bowl when the float valve shuts off the fuel flow from the tank. Measure the distance on your carburetor, and then make a scratch on the side of the float bowl at the proper fuel level. Turn the fuel cock ON and hold the neoprene tube up beside the float bowl. The fuel in the tube should rise to the scratch mark. If it is too high, the float level must be raised. If it is too low. the float level must be reduced.


The procedure for adjusting the idle speed is covered in Chapter 2, Tuning for Performance. The carburetor settings listed in the specification table at the end of this chapter are the manufacturer’s recommendations for general usage. Because conditions of operation may differ, it may be necessary to experiment with the carburetor adjustments and tuning to obtain peak engine performance and/or best fuel economy. This section explains how to tune the carburetors for each mode of operation. Before changing the jetting of the carburetors, be sure the ignition system is in good condition and the engine is properly timed. The carburetors must also be properly synchronized.


In order for the engine to run smoothly and deliver the best performance and fuel mileage, all three carburetors must act together. They must be synchronized so that all three throttles lift the same amount at the same time, so that all three pilot systems and main systems are working in unison.

To synchronize the carburetors, first warm the engine to operating temperature, and then switch it off. Loosen the throttle cable adjuster at the twistgrip to get as much cable slack as possible. This moves the sliding block that carries the four lower cables (one to each carburetor and one to the oil pump) all the way to the bottom of the cable junction box. Shorten the cable adjusters on the carburetor caps all the way. Remove any cable clips from the adjusters.

Now remove the air pipes from the mouths of the carburetors. Set all three air screws to the setting recommended in the specification section at the end of this chapter. Lower all three throttle slides as far as they will go, by turning the throttle stop screw or adjuster. On H2’s, S2’s, and S3’s, turn the throttle stop screw counterclockwise; on H1’s and S1’s, clockwise. Feel with your fingers or use a mirror to see that all three throttle slides are at the bottom of their travel. Turn each throttle stop in the opposite direction until each slide just begins to lift, and then make one additional turn. This will synchronize all three carburetors at a slow idle.

Start the engine. If it will not run, turn each throttle stop exactly one more turn to speed up the idle slightly. To increase engine idling speed to specifications, turn all three throttle stops 1/4 turn at a time in the same direction, until the idle is constant at 1,100 to 1.300 rpm.

If you have access to a Uni-Syn or similar air-speed sensing tool, hold it against the mouth of each carburetor in turn and adjust the throttle stops until the ball is lifted the same height on each carburetor. Then turn all three throttle stops 1/4 turn at a time in the same direction until the idle is constant at 1,100 to 1,300 rpm. Switch off the engine.

Lengthen each cable adjuster on the carburetor cap until the cable sheath has 1/16″ free play. Now turn the cable adjuster at the twistgrip until the grip also has 1/16″ free play. While turning the twistgrip back and forth. check with your fingers or a small mirror to be sure that
all three throttle slides start to lift at exactly the same time. Replace the air pipes and any dust covers and cable clips that were removed.

There are a couple of alternative carburetor synchronization methods offered below that may offer greater precision:


1) First back off the idle screws until they don’t touch the slides.

2) Carefully screw each one in until the screw just barely touches the slide.

3) Turn in each screw the exact same amount, until you get your target idle number.  If you don’t do this first, the little variance you get when setting the idle screws will affect slide height and the sync will not be “spot on”.

4) Make sure you have slack in the cables.

5) Put your middle finger of your left hand on the center slide, and your thumb (left hand) on the right slide.  Turn the throttle very slowly and feel if the slides lift at the same time.  If not, adjust one or the other cable so they do.

6) Snap the throttle a couple of times to make sure the slides are setting in well, and tighten the cable lock nut and recheck.

7) Move your thumb to the center slide and your middle finger to the left carb.  Adjust the LEFT carb till it lifts exactly with the center.

8) Snap the throttle again and make sure the lock nut is tight (tightening the lock nut will change the slide height).

9) Open throttle until slide is even with top of carb throat.  Feel that all slides are at the same position.

10) Take out any extra slack in the cable, AND check the oil pump for correct setting.

The finger method can tell movement in thousands of an inch (just say very accurate).  Set the sync from idle, because that is where it is most important.


1) Find a smooth round pin about 3/8″ or 10mm dia. (the shank of drill bit works well).

2) Remove air box/filters.

3) Back out slide stop (idle adjustment) screws.

4) Set throttle lock or set throttle adjuster at the grip so the pin will just lightly drag as it is inserted in the carb throat under the slide cutaway of one carb.

5) Set the other carbs so they offer the same resistance when the pin is inserted by setting the cable adjuster at top of each carb.

6) Release throttle lock or reset throttle adjuster at grip insuring that slides on all carbs will fully bottom out and throttle grip has 2-3mm play.

7) Set air and idle adjustment screws for best idle.

As a final check to insure all idle adjustment screws are set the same, insert a nail, spoon, or long toothpick under each slide without altering slide position.  As the grip is turned the ends of all three should tip at the same time.  Readjust idle screws as required.


To tune the carburetor properly for idling and low-speed running, you will have to adjust the pilot system. The principal adjuster of the pilot system is the air screw. First, set all three air screws to the specification given at the end of this chapter. Now synchronize all three carburetors and set the idle speed, as described above and in Chapter 2, Tuning for Performance. With the engine idling, turn all three air screws in or out 1/4 turn. Listen to the exhaust and note any change in the firing pulses. Place your hand one inch from the ends of the mufflers to feel the exhaust pulses. Turning the air screws clockwise makes the mixture richer, turning them counterclockwise makes it leaner. If the engine begins “four-stroking,” that is, firing on every other stroke instead of on each stroke, the mixture is too rich. If the exhaust note is very uneven or irregular, the mixture is too lean.

Some other signs of an excessively lean idle mixture are hesitation and poor throttle response when accelerating from idle, overheating when the bike is ridden at slow speeds, heavy detonation when the bike is ridden at highway speeds, a marked idle speed increase (more than 300 rpm) when the engine is hot, and having the engine take a long time to idle down after a high-speed run.

Some signs of an excessively rich idle mixture are four-stroking and sputtering at an idle, fouling the spark plugs when riding at slow speeds. and excessive fuel consumption.

Generally speaking, for better gas mileage and smoother running around town, turn the air screws out 1/4 turn from the specified setting. unless detonation is evident at highway speeds. For better throttle response, better low-end torque, and easier starting on cold mornings, turn the air screw in 1/4 turn, from the specification. Of course the standard setting is given in the specification table.

The final idle mixture adjustment should be no more than 1/2 turn from the specified setting. If it is, check for a clogged pilot jet, a restricted pilot air channel, an obstructed low-speed outlet in the carburetor throat, or an air leak at the carburetor mounting spigot or flange. NOTE: Turning the air screw has an effect similar to changing the size of the pilot jet. If the best air screw adjustment is more than 1/2 turn from the specified setting, the pilot jet should be changed instead. If the air screw is 1/2 turn (or more) clockwise from the recommended setting, change the pilot jet for one with a number that is five higher. For example, if the carburetor has a #25 pilot jet standard, replace it with a #30. If the air screw is 1/2 turn (or more) counterclockwise from the recommended setting, change the pilot jet for one with a number that is five lower, i.e., #25 to #20. There is a listing of available pilot jets and their Kawasaki part numbers at the end of this chapter in the specification section. CAUTION: Don’t lean the pilot mixture enough to cause detonation at highway speeds. Detonation will cause extensive damage to the pistons, rings, crankshaft bearings, and spark plugs.

NOTE: Each carb must be ADJUSTED for optimum idle via AIR SCREW adjustment…. seeking the point where idle rpm for that cylinder is highest. That is the point where the fuel/air mixture is optimum at idle rpm. Starting from scratch, unless you’re extremely lucky, there is no “balance” between cylinders or carbs…. one cylinder will be pulling the other two. When this condition exists ONLY the carb on the pulling cylinder will respond to adjustment. Setting the idle stop to insure that the “pulling” cylinder carb is in control of idle rpm will then allow adjustment of that carb to be seen in rpm changes. An alternative is to pull the plugs of the cylinders not being adjusted so it would be apparent which cylinder is “pulling”. If, using this method, an air screw has no effect on idle speed, something is wrong.


The fuel mixture in the midrange mode is changed by moving the clip on the top end of the jet needle. For most usage the standard clip position is best. The grooves in the top end of the needle are numbered from top to bottom, #1 to #5. For high-altitude riding, the needle may be lowered to lean the mixture by moving the clip to a lower-numbered groove; for instance, from groove #3 to #2. For riding in cold, damp weather at sea level, the mixture may need to be enriched for best running by raising the needle; for example, moving the clip from groove #3 to #4.

If the engine hesitates and/or backfires when accelerating from 1/2 throttle, the midrange mixture is too lean and the jet needle should be raised (move the clip to the next-higher-numbered groove). This will allow more fuel to flow between the jet needle’s tapered section and the orifice of the needle jet.

If the engine is sluggish and stutters when accelerating at 1/2 throttle in high gear, the midrange mixture is too rich. The jet needle should be lowered to restrict the orifice of the needle jet. This reduces fuel flow (leaner mixture) for an equivalent throttle opening. Take the clip out of its present groove and move it to a lower-numbered groove. CAUTION: Do not lean the midrange too much or detonation will result.

If the midrange mixture is not satisfactory after adjusting the jet needle, check to be sure that the needle jet is tight in the carburetor body, that the float level is correct, that the primary air passage is open, and that the jet needle clip is in place. If the engine has over 10,000 miles on it, check the center section of the needle for wear. If it is shiny, it has worn against the needle jet because of engine vibration. Both the needle and the jet must be replaced to guarantee like-new performance.


The fuel mixture at high speeds and large throttle openings is controlled by the main jet. NOTE: The main jet is not effective until the area between the end of the jet needle and the inside of the needle jet is greater than the area of the main jet opening. The size of the main jet is marked on it. The number is a code for the diameter of the opening in the jet; the larger the opening, the higher the number and the richer the mixture at full throttle. All Kawasaki triples use reverse-type main jets. They have round heads with a screw slot. CAUTION: Do not use hex-headed main jets in these carburetors because the threads are different, which will strip the threads in the needle jet.

To test the main jet, accelerate momentarily at full throttle in high gear at about 50 mph. If the main jet is too small (lean) or too large (rich), the engine will not respond well at full throttle and will regain power only when the throttle is closed to the 3/4 position (which reactivates the midrange system).

If the main jet is too large, full-throttle performance will be sluggish and the exhaust note will be stuttering. Inspection of the spark plugs will show a dark brown or sooty black color on the insulators. NOTE. These indications can also be caused by too cold spark plugs or retarded ignition timing. Install a main jet with the next size smaller number for a leaner mixture. Check the list of main jet sizes and part numbers at the end of this chapter in the specifications section. If there is still no improvement. check for a dirty air cleaner, an obstructed air cleaner inlet, or clogged muffler baffle tubes.

If the main jet is too small. the engine may backfire or hesitate and accelerate in lurches when the throttle is opened fully. The spark plug insulators will be white or grayish white. Too lean a mixture will cause overheating, and if the condition is excessive, small flecks of aluminum will be evident on the spark plug insulators. NOTE: These indications can also be caused by too hot a spark plug or overadvanced ignition timing. A main jet that is too small will cause detonation at full throttle which sounds like static electricity. It is not the same as the “pinging” sound made by an automobile engine running on too low an octane rated gasoline. CAUTION: If detonation is heard at full throttle, back off the throttle immediately or major engine damage will result.

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