Group K Modifications for Yamaha 2-cycle Jet-boats 

  Through the late1990s Group K Yamaha 3-cylinder PWCs dominated most of the professional racing classes in PWC Endurance racing.  Given our intimate familiarity with the Yamaha triple engines, we also undertook development of performance modifications for the Yamaha jet-boats that used these same engines.

  Our development netted some impressive gains in performance (with no compromises in reliability).  However the entire process taught us one very valuable lesson ….The Yamaha jet-boats are very different kinds of boats with needs that are very different from that of a PWC.  We found that the mods that work perfectly on our PWCs do not necessarily work so ideally on a jet-boat, despite having the same engines and jet-pumps.

  This document will outline the needs we found most important for the various Yamaha jet-boats, and how we addressed those needs in our process to get better performance.

   The primary focus of all our jet-boat development was to make the maximum increase in overall acceleration along with improved pump “hook-up” …… and there were huge gains to be had in both areas.  It bears noting that NO pump gas compatible kits for any of these Jet boats can make “huge” increases in peak speeds.  Since they all have so much wetted hull surface area, and no effective trim system, our maximum speed gains over stock were usually on the order of 4-5 mph.  Seeing peak-speed gains greater than that would have require the use of 100+ octane fuels, as well as some considerable compromises in long term reliability.



Stage 1

This modification involves an 89 octane safe cylinder head modification to attain better overall acceleration with a slight increase in peak speed.  Stock props can be retained, but aftermarket props are recommended.

Stage 2

This modification involves a 91 octane safe cylinder head modification to attain much better overall acceleration along with an increase in peak speed.  It is strongly recommended to use aftermarket impellers.


This is the highest level of 91 octane safe performance available.  This kit offers huge increases in overall acceleration, and mandates the use of aftermarket impellers.


An overview of the Stock Yamaha Jetboat Models






Dry Weight

Fuel Capacity

Exciter 135


1131cc / 135hp x 1



35 gall

Exciter 220


1053cc / 110hp x 2



35 gall

Exciter 270


1131cc / 135hp x 2



35 gall

LS 2000


1131cc / 135hp x 2



41 gall



1176cc / 155hp x 2



41 gall

AR 210 2003-2004

1131cc / 135hp x 2

LX 210 2003-2004

1131cc / 135hp x 2



Exciter 135

In stock form, the single engine 135 is barely fast enough to be entertaining.  The stock single jet pump struggles to operate without cavitation.  When put into higher speed turns, the cavitation is so pronounced that it compromises steering ability.

  All that said, the 135 gains a greater benefit in overall performance from our modifications than any of the jet-boats we tested.  With our Skat Trak Swirl Impeller, and any level of engine modification, the 135 becomes a very fun and useful cruiser.  With it’s single engine and 35 gallon tank, the modified 135 has a much longer fuel range than any other jetboat in the line (about 70 miles @ 35-40 mph).   The Sleeper equipped 135 turned out to be such a great balance of destination cruiser and sport-toy that we kept it on as one of our permanent in-house fleet.

Exciter 220

  All our test drivers found the Group K modified 220 to be the most practical balance of power that is matched to the dual 155 pumps.  Since it has 38mm carbs (instead of the 44mm found on the other twin motor models) it offers the best fuel range of all the twin-engine models.  The modified 220 offers the same “strong and stunning” authority of acceleration off the turns as the 270, but with much better fuel range.  With overall acceleration and peak speeds that are not far off the 270, the modified 220 is the pocket rocket for the practical minded jet-boater.

Exciter 270

If you want the fastest, quickest, and most vicious of all the Yamaha jet-boats…this is it.  The modified 270 accelerates off the turns with an authority that can yank off your sunglasses, and cause your passengers to “hang on for dear life”.  The modified 270 can still cruise nicely, and easily pull any tow toy you have.  But if you decide to run it pinned, it is a serious water weapon that demands your undivided attention….The Sleeper 270 delivers response and excitement that is unmatched by any other jetboat.


The LS has exactly the same engines and pumps as the rocket ship 270, however the added weight and hull length makes the LS a much tamer machine.  The added workload means the stock LS experiences quite a bit more cavitation than the 270, and markedly less peak speed.  The modified LS has the improved acceleration, and great hook up, that this boat needs for family cruising and towing sports.

XR 1800

The stock XR engines make considerably more power than all the other models, however it still drives through the same 155 jet pumps as the rest.  As a result, the stock XR struggles with controlling pump cavitation.  Despite it’s power advantage over the 270, its extra weight and wetted hull surface area nets acceleration and peak speeds that are questionable better than the 220, with much heavier fuel consumption.  That said, the XR is better than the Exciter boats when it comes to carrying gear and multiple passengers.  With all its power potential it seems the XR is geared more toward towing sports.  Our mods for the XR are geared more toward hooking up the power being made, and generating a kind of power increase that the pumps can effectively deliver.

Dealing with the Common Problem Areas First

Getting reliable performance increases from the Yamaha Jets boats is not difficult, so long as the biggest problem areas are attended to first.  After months of testing with the Yamaha jet-boats, we found two significant problem areas that required this level of attention.  Specifically, these areas were the jet pumps, and the fuel systems.  Those problems and solutions are outlined below.

Jet Pump Issues

All the Yamaha jet-boats use 155mm pump assemblies that were originally designed for use in PWCs.  The 155mm refers to the outside diameter of the impeller blades.  The thrust generated by these 155mm pumps is more than enough to provide the lightweight PWCs with instant acceleration from a dead stop … all the way to peak speed.  In addition, the PWCs experienced very minor losses in pump efficiency when they were put in to high-speed turns.

  Unfortunately, the jet-boats are about 3-4 times the weight of a PWC, and have a whole lot more “wetted” hull surface area that offers heavy resistant to the pumps.  It’s much like using a street-bike engine and tire to push around the mass of a large automobile.  The contact patch of a motorcycle tire could be “adequate” for most uses, but it’s traction abilities would be constantly stretched to the limit.

  In the case of the Yamaha jet-boats, the use of two pumps on the twin engine machines offers a huge advantage in delivering the boat’s power, but even the twin pumps of these jet-boats are often stretched to their limits by the loads of sheer weight and wetted-hull surface.  Pump thrust improvement has been our top priority in our jet-boat testing, and is addressed in detail below.

Some Jet Pump Basics

What is Cavitation -  “Impeller Cavitation” takes place when the impeller is processing water out the back of the jet pump faster than new water is being admitted in  to front of the water intake grate.  When this happens, the prop will begin spinning in aerated water, causing big loss of jet thrust…it gives the driver a feel much like a slipping clutch on an automobile.  Cavitation has two very bad side effects.

  First and foremost, is the efficiency of the cooling system.  The coolant water being supplied to the engine on all Yamaha jet-boats is delivered from the pressurized chamber in the pump case(s).  When the prop is cavitating, the cooling water being delivered to the engine is full of air bubbles that offer no cooling benefit at all.  A few moments of getting “foamed” water to the cooling system doesn’t represent any big problem.  However, if a prop is operated in a cavitating state for an extended period of time, the engine can easily experience serious overheating.  Doing full throttle doughnuts keeps the impeller in a full-time state of cavitation.  Doing this may be fun and exciting for some owners, however the "coolant starved" engines in your jet boat don’t like it one bit.

  Beyond the negatives of the cooling system, cavitation within the pump causes some other destructive forces that can damage the prop and pump housing.  Excessive cavitation can begin to actually erode material away from the blades of a stainless steel impeller (see photos).  As this damage takes place, it fosters more and more cavitation.  If cavitation goes on long enough, the damage to the impeller blade can result in a fractured or broken impeller blade.

  It’s virtually impossible to operate any of the Yamaha jet-boats in a way that “never” produces cavitation, however the objective is to drive your jet-boat in a way that minimizes prop cavitation.

What is "Prop-Slip"

In general terms "prop-slip" is a milder version of cavitation and the water equivalent of mild wheel spin on a ground vehicle.  Prop-slip is different from cavitation in the sense that it takes place when a rapid and solid supply of water is still entering the pump intake.  During prop-slip there is still a strong output of thrust, but that thrust is by no means cavitation free.  Because prop-slip takes place while pump pressures are high, the "cavitation erosion" to the impeller is different.  The aftermarket Skat Trak and Solas impellers have improved hub and blade designs that help reduce prop-slip, but they cannot eliminate it.

  Prop slip most often takes place when a jetboat is put into a very high speed turn.  With our digital tachometers, we watched our engines revving about 6900 rpm in a straight line on glass water.  As we snapped the wheel into a full lock turn, we watch the tach instantly registering 7050 rpm as speed declined appreciably (indicating that prop-slip was taking place).  If we snapped the wheel immediately in the other direction, we saw the engines still at 7050rpm, but our corner speed dropped even more.  If we attempted a long series of full throttle "S" turns, the corners speeds dropped and dropped with each successive turn until eventually the boat was below planeing speed, and the pumps were operating in full cavitation.

  In short, the larger hull boats are much more prone to prop-slip, and the physical damage to the impellers that it produces.  Owners who routinely run their XR1800 and LS2000 jet-boats through full throttle turns should be aware that this kind of operation (while fun) does take a toll on the long term life of the props.

Note "slip" damage shadows on outer edges Close up view of "prop-slip" erosion  damage to stainless steel impeller blade  

About Pump Loading

  All technicians that build high performance PWCs quickly learn a few fundamental rules about PWC pump function.  As you engage a jet-pump driven watercraft in to a high-speed (power-on) left hand turn, the engine/pump gets very heavily loaded, and engine rpms are usually “pulled down”.  However, making a right hand turn at exactly the same speed results in no rpm loss at all … and often results in an rpm increase, often accompanied by mild cavitation.

  The reason for this phenomenon is that the entry angle of water coming in towards the impeller has a big effect on how well the pump get “loaded”.  In left hand turns, the water comes toward the prop at an angle that very effectively “loads” the pump with water, and allows for very minimal “cavitation”.  A hard right hand turn has exactly the opposite effect because the water is coming in toward the prop in a way that is very “unfriendly” to pump loading.  Anyone who has driven a single engine Exciter 135 will experience this phenomenon in a big way.  Using aftermarket impellers with a better blade/hub design can greatly reduce the cavitation experienced under all conditions …including in right hand turns.   But even the best designed prop cannot eliminate right-turn cavitation altogether.

Twin Engine Pump Loading

 The phenomenon described above affects the twin motor jet boats in an entirely different way.  The water intake surfaces on the bottom of twin motor hulls are on angled surfaces on each side of the hull.  This “angled water entry” gives an effective entry angle that mimics the pump loading of turning the boat.  This means that (while driving in a straight line) the drivers side pump (starboard) is receiving water at an angle that mimics a left hand turn (thus loading the engine rpms down).  At the same time, the passenger side (port) impeller is receiving water at an angle that mimics a right hand turn (resulting in higher rpms and a closer cavitation threshold).  This is why the left (port) motors of most twin-engine Yamaha jet-boats tend to run higher rpms than the right (starboard) engines.

  Getting past the whole theoretical aspect, the functional result is what’s important.. Because of the phenomenon described above, the right side pump is generating more actual thrust than the left pump at any given rpm.  This causes the steering wheel to constantly be “tugging” toward a left hand turn, and at the same time causes the left side engine to rev higher than the right engine in order to generate the same thrust.

  After weeks of testing, we eventually resolved all these problems (on twin motor Yamahas) by fitting better design impellers on the pumps, and applying a noticeably steeper pitch to the left hand side prop.  The end result is engines that turn virtually identical rpms all the way through the throttle movement range, and an end to the steering wheel constantly “tugging” to the left.  In most cases, the “off-the-shelf” pitch of the props we chose were not right on the money, so we custom pitch each pair that we sell to the specs that worked best during our on-water tests.  Installing these “staggered pitch” aftermarket impellers is one of the most effective improvements that you can make to your twin motor Yamaha jet-boat.

About the Jet-Pump Cleanouts

  Yamaha built all their Jetboats with a “debris cleanout” access plug over the jet-pump intake.  This cleanout is a great convenience for owners who drive in areas where there is lots of sea-grass or other floating debris that can make it’s way into the jet pump inlet.

  Unfortunately, some owners have experienced high-speed episodes where the cleanout “plug” is suddenly ejected from it’s position over the pump inlet.  When this happens, the plug often strikes the access door with such force that it gets pushed open.  When the access door is pushed open, the pump plug is often blasted into the air, and vanishes into the open water.  As the access door is blown open, it actuates an engine cutoff switch.  This creates a situation where the boat decelerates rapidly, while a large column of water is being driven upward through the opening in the top of the water intake.  In such a situation, most of that water column will fall into the passenger compartment of the boat…. And pretty much soak everyone.  To add to the insult of all occupants taking an instant dowsing … the pump plug is gone overboard, thus rendering the boat close to inoperable (actually, twin motor boats can safely be driven at low speeds on one engine with a pump plug gone)

  The first time this happens is enough to “motivate” most owners to seek out a permanent solution (us included).  We spoke with the Yamaha technicians about this problem, and they embraced the consensus that access plug ejection  took place as a result of pump cavitation.  After experiencing this episode many times during our own testing, we couldn’t disagree with them more.

  “Every” time that we experienced “plug ejection” was while moving at high speed in very rough water.  Under these conditions the pump is repeatedly being loaded and un-loaded at very high rpm (creating a situation water pressure repeatedly goes from zero to max).  It is our belief that the plug is ejected from the sudden water pressure of an unloaded pump that becomes suddenly loaded with water while the engine is at higher rpm.  We “never” experienced a plug ejection during a time where the prop was cavitating.  Regardless of the disagreement of the theory of the causes, we feel very clearly about the best solutions.

Our Solutions

  First and foremost, a float must be attached to the pump plug and it’s hardware.  We accomplished this by threading a nylon rope through all the parts involved, and attach them to a float .  This assures that you can, at the least, can retrieve the parts to re-install them and get back underway.  While this float attachment “is not” a solution, we consider it a wise precaution that every Yamaha jetboat owner should consider.

  After much testing with many different approaches to solve the ejection problem, we chose a two-part approach that worked great for us.  In short, we glue in the cleanout plug in place with a heavy application of Permitex Industrial clear silicone, and then install an aftermarket impeller with a radiused leading.  In the event that the siliconed-in access plugs might ever “have to” be removed for maintenance, the Permitex silicone can be released at any time by pouring some cleaning solvent, kerosene or gasoline down the port holes (but we never had any need to remove ours). 

   One would think that the blades of the stock impeller could easily cut any sea grass that gets near it, but unfortunately that is not the case.  Sea grass simply lays over the leading edge of the stock design impellers and clogs the pump.  With the aftermarket “radiused leading edge” props, the grass is swung to the outer edge of the blade where it is cut between the radiused edge of the prop and the outer impeller housing.  Even with this setup, we have occasionally loaded our pumps up with grass, however we were consistently able to clear the grass away by driving the boat at low speeds and then cutting off the motor (hence washing the grass out the front of the pump).  Any remaining grass was then easily processed through by the new prop. 

  While we don’t consider this a totally perfect solution, the minor inconveniences were well worth the confidence of never taking “the big bath” again. 



Bungee cord ends, fitted with a nylon rope, attach to a float to keep from losing all the essentials. The Installed Setup.  Not pretty, but it works great.




About Impeller Choices

There is much performance to be gained from all the Yamaha Jetboats by converting to aftermarket impellers.  During our testing we tried many different designs and pitches.  At the end, there are two top choices that were clearly best.  Please note that impeller pitch selection is a function of the power output of your engine setup, and relies on the presumption that you have a well sealed pump shoe and good condition impeller housing.  Also note that “all” impeller makers have their own means of measuring and stating pitch angles.  This means that a 13/19 from one manufacturer will have completely different engine loading characteristics than a 13/19 from another manufacturer.  Pitch angle comparison is “only” relevant so long as the brand and hub contour design is the same.  At Group K we have tested all the popular impellers to find the correct brand/pitch for our engine packages.


Stock Jetboat Impeller

Solas “Concord” – The best top speed & much improved hook-up

Skat Trak “Swirl” – The very best hook-up with a slight speed loss

We consider the Solas “Concord”, and Skat-Trak “Swirl” props to be the best choices for any Yamaha Jetboat.  The Skat Trak  “Swirl” was our favored choice for the single engine Exciter 135.  With this prop, the 135 had excellent acceleration at all speeds, and drove off the turns with outstanding authority.  While this prop does not net quite as much peak speed as the Solas Concord, it offered initial take-off and turning control that was far superior to the best Solas we tested.  Sadly, the 135 is very sensitive to correct pitch angle, and the Skat prop requires pitch adjustment to work perfectly on the 135.  During our testing, we found the ideal pitch of the swirl in each 135 engine modification level, and we will spec each prop to your application.

About Twin Motor Props

  While the Skat Trak Swirl is the clear best choice for the 135, the twin engine boats have more options.  By sheer design, the twin pumps of these models offers a huge improvement in initial take off, and strong thrust while turning.  Given that, we usually opted to use the Solas Concord props instead of the Skat Swirls.  The Solas props have a considerably smaller hub contour that allows them to literally process more water (per revolution) than the other props.  The result is that the Solas actually generates more thrust (and speed) per impeller revolution than the stock prop.  Since the Solas prop has less blade surface area than the Skat-Trak swirl, it nets better peak water-speeds than any of the props while still offering a huge improvement in hook-up under all conditions.  In addition, the Solas leading edge is radiused enough to effectively cut incoming sea grass (as the Skat Swirl does).  On modified jet-boats, the Solas can still experience "some" prop-slip in high speed turns ... but still alot less slip than the stock props.

  For owners who are not too concerned about peak speeds, but desire the very best in resistance to cavitation and prop-slip... the Skat-Trak swirl is the very best.  In particular, LS2000 and XR1800 owners who do lots of towing sports or rough water "shredding", the Skat Swirls should be strongly considered.

Fuel System Issues

Engine Starting -  Since PWCs routinely get upside down in the water, the Coast Guard regulations have always mandated that they have a fully sealed fuel system.  The PWC makers have used this mandate to their advantage.  As a pwc fuel tank gets agitated, a small amount of internal pressure builds (exactly as shaking a 5 gallon can of fuel results in built up internal pressure).  In PWCs, this natural pressurization is utilized to help feed fuel through the fuel lines to the carburetors.  Furthermore, when a PWC is shut off, this sealed fuel system holds fuel in place where it is immediately available for the next restart.

  Ironically, Coast Guard regulations “”mandate” all traditional pleasure boats (including the Yamaha Jet-boats) utilize an un-sealed “vented to the atmosphere” fuel system.  Overall, such a system can work okay, however it does represent one big inconvenience … hard starting. 

  In the “unsealed” system used on the jet-boats, most of the fuel in the fuel lines is often siphoned back into the gas tank when the engine is shut off for a few hours.  In order to re-start, the fuel pumps on the carbs must re-fill the long empty fuel lines in order to get the engine to fire.  This often requires lengthy runs on the ignition starter key.  Since the fuel lines to the port (left) engine is a longer run, the port motors usually take alot more cranking to get their fuel.  Since the starters are not designed for long usage, these lengthy starter uses (to re-fill the fuel lines) can cause premature starter motor failures.

Air in the Fuel Lines

  High performance PWCs (and jet-boats that use pwc engines) all have a shared problem.  As fuel level gets low, air bubbles routinely make their way into the fuel pickups and then into the fuel lines. Since there is no way for this air to be purged out (once it has entered) the air is passed through the carburetor jets where it causes a momentary, but serious, lean condition.  As more and more bubbles are admitted, these moments of lean condition becomes so prevalent that the operator experiences a noticeable “bog” or hesitation.

  On low performance pwc engines, these “lean moments” are of little consequence.  However on high performance PWCs (particularly 3 cylinder engines), these lean moments often result in momentary detonation, and “more than occasionally” result in piston scoring or piston seizure.  Countless times, owners of high performance PWCs and jet-boats have experienced a sudden unexplainable engine “shut down”.  Examination afterwards shows the shut-down was the result of a scored piston … but why??

  Our testing showed that in many cases these mysterious scored-piston shut-downs are a result of a “more than momentary” lean condition being caused by air entering the fuel pickups when fuel levels are below 1/2 tank.  To resolve this problem, we created a fuel/air separator kit that assures an “air free” supply of fuel to the carbs at all times.  This kit can be easily fitted to most PWCs and jet-boats, and brings with it improved starting abilities reduced carburetor maintenance, and total elimination of all air bubbles in the fuel lines.

About Air in the Fuel Lines – The Technical Background

  We purchased a single motor Yamaha Exciter135 jet-boat for performance increase tests.  After spending a few weeks of testing with various props, compression ratios, porting formats, etc, we got down to doing the carburetor fine-tuning.  Like all our fine-tuning tests, the boat was equipped with a gps, digital tachometer, and a detonation sensor that had a lead to each cylinder.  We were in the second day of fine-tuning, and the boat was operating perfectly, even under the heaviest high speed loads we could inflict upon it.

  Mid way through our test session, another boat we had at the water needed fuel.  We pulled up the jet boat (with it’s 35 gallon tank about half full) and siphoned out 6 gallons to put in our other boat.  We figured we still had at least 12 gallons of fuel on board, which would easily be enough to complete our testing.  With the siphoning done, we headed our Exciter jet-boat back out on the water to finish our fine tuning.  Immediately, we started seeing mild detonation on all three cylinders as the jet-boat ran across the bumps of some small boat wakes.  We then ran the boat to peak speed for our standard “full speed left turn” test (this test loads the engine harder than any other kind of operation).  As soon as we snapped the wheel to full left, all three cylinders showed heavy detonation that would have easily scored a piston if we maintained the turn for a couple of more seconds.  We were stunned that this engine, that had been performing flawlessly for days, was suddenly on the verge of burning down a piston.  We checked our entire engine compartment to make sure that some random component failure or fuel restriction wasn’t causing this new and very lethal detonation.  After exhausting all possibilities, we went back to the launch ramp and added back the 6 gallons of fuel we had just siphoned off.   Back out on the water, our jet-boat once again performed flawlessly with no visible detonation, even under the heaviest high rpm operation and hardest turns.

  For us, the message was clear.  The fuel in the long and narrow fuel tank of our jet-boat could easily be ”sloshed” away from the fuel pickup tubes.  Once the pickup ends are exposed to air, that air enters the fuel lines.  Our jet-boat (like all pwcs) has no way to purge out this air in the fuel lines, other than passing it through the jets in the carburetors.  As soon as these air bubbles reach the jet circuits, they create a very brief, but very serious, lean condition.  These brief lean conditions were causing the mild detonation our jet-boat exhibited as we drove across the bumpy boat wakes.  However, our full-speed left-turn sloshed virtually all the fuel to one side of the tank, causing an instant and very lengthy lean condition.  A lean condition of this order can easily score a piston in less than 3-4 operating seconds (under load).

The Solution

  We decided to create a fuel/air separator for our project jet-boat to purge all air bubbles from the fuel lines “before” they reached the carbs.  “All” fuel/air separator setups require the use of a remote fuel pump.  We tested with both electric and “pulse” type fuel pumps, and various separator chamber designs.  Through the whole process we maintained a “keep it as simple as possible” mindset.

  We finalized on two separate designs.  (A) The crankcase “pulse” fuel pump kit, and (B) the electric fuel pump kit.  The electric fuel pump kit is intended primarily for the twin-engine jet-boats (Yamaha and Sea Doo), however it could be used on any pwc with a strong charging system as well.  The “Pulse” fuel pump Kit is the more simple setup intended for PWCs, and it does mandate the installation of a primer pump (or carbs with an accelerator pump feature to prime the carbs).

  After installing our finished separator on our Exciter 135 test jet boat, we re-ran our tests with only 5 gallons of fuel in the tank (the earlier tests showed heavy detonation with 12 gallons on board).  This is a level that would have certainly delivered heavy detonation during high-speed rough-water operation, and piston seizing detonation during our “full speed left turn” test.  No matter how hard we ran our test jet-boat with this low fuel level, we never experience even the slightest detonation, or “low fuel” hesitations.  The fuel/air separator was obviously doing a perfect job of purging all the air from the fuel input lines to the carbs.  With this separator in place, we could run the fuel level down to virtually zero without experiencing any “engine harming” detonation or lean conditions.

  We consider this fuel/air separator to be absolutely mandatory for any Yamaha jet boat that gets operated in any fashion with less than a half tank of fuel.

  In addition to the above mentioned benefits, we also found that our hard-starting problems were also eliminated.  Our electric fuel pump (which was wired in to the ignition key) would immediately fill the fuel lines that had been siphoned empty, and our Exciter started almost instantly.


The Yamaha Jet-boat fuel/air separator consists of the electric fuel pump, separator canister, and all the hoses & fittings required for installation.

(note: electrical wiring for connection to key switch is not included)


 About Using PWC modifications on Jet-boats

 There are many performance shops (including Group K) that make and sell performance modifications for the Yamaha 1200 pwcs.  Some of these shops infer that the modifications that work on pwcs will net the same improvements on the Yamaha jet-boats..... and we generally is why.  One of the biggest needs of high performance PWCs is the need to respond and accelerate instantly (especially from a dead stop).  Since PWCs are so much lighter, the pumps can easily generate the instant thrust and acceleration without experiencing much cavitation.  In addition, pwc trigger throttles allow a rider to instantly draw the throttle from full closed to 100% in just an instant.  Engine and pump modifications that improves a PWC's ability to perform this "instant" acceleration don't offer the same benefits on jet-boats because the pumps of the jet-boats are not capable of delivering instant thrust from a dead stop, and the throttles of a jet-boat cannot be worked nearly as quickly as the throttle on a PWC.  Additionally, jet-boats are very often cruised at a fixed partial throttle setting .... and PWCs are seldom operated in that way.  These different operating characteristics makes for different reactions to many modifications.

About Compression – The Yamaha Jet-boats respond very well to increases in compression … within limits.  Our Stg 2 Kits, and Sleeper kits, have been developed to run the highest compression ratios that we consider to be 91 octane pump-gas safe.  We attain this higher compression by machining the stock cylinder-head (a much more affordable alternative to buying another head).  After milling the stock head, we also re-cut the domes for a shape that better staves off detonation.  It's important to note that the three cylinders of the Yamaha engines have differing operating temperatures.  In order to help equalize these temperatures, we slightly "stagger" the compression ratios of our modified heads.  This staggered compression helps greatly to reduce the detonation risks of the higher temp cylinders.  To our knowledge, there are no aftermarket bolt-on heads that provide this 'staggered" compression feature.

  The specification of the Stage1 and Sleeper modifications are different, however the Stage1 modified head can be re-cut to the Sleeper spec.  It also bears noting that our jet-boat head specifications are very different from the head mods we do for the same models of PWCs,  and should not be interchanged. Since the engines in the jet-boats are being loaded so much heavier than the same engine in a PWC, the compression ratios need to slightly milder in order to assure safe operating temps.

   Because of variations in starter cranking speeds, compression gauges, and measuring techniques, we do not specify the compression of our different arrangements in terms of “psi” pressure readings.  Our technicians do not discuss any compression specifications of these kits in terms of psi or ratio … please don’t ask them to.

About “O” Ring Cylinder Head Sealing – Some performance shops manufacture “billet” replacement cylinder heads known as “O” Ring Heads.  These heads are manufactured to utilize a large diameter rubber “O” ring to seal between the cylinder and head (instead of the stock metal head gasket).  This “O” ring system can offer great convenience to owners who regularly remove the head for the purpose of inspection or compression changes (in the case on changeable dome heads).  The other big plus to the “O” ring setup is that it can significantly reduce squish clearance without having to machine the top surface of the cylinder.  This “O” ring system is the primary means of head sealing on many stock Sea Doo and Polaris PWC engines. 

    It should be understood that an “O” ring can easily seal water away from the cylinder bores, however no “O” ring on earth is strong enough to seal back the pressures of compression.  “O” ring setups depend on a relatively broad metal-to-metal mating surface around the bore diameter to seal compression.  The absolute flatness of the surfaces on the head and the three cylinders is fundamental to maintaining a lasting long-term seal.  Herein lies the problem with some “O” ring head setups.  The three cylinders must have perfectly flat (preferably lapped) surfaces on their top “head sealing” surface.  If the sleeves in a cylinder have dropped very slightly in the aluminum casting (as they frequently do, an "O" ring had has no way to maintain an effective seal.  The stock metal head gasket has the ability to more effectively accommodate all these possible surface variations.  For this reason, all Group K head modifications and engine kits are prepared to utilize the stock metal head gasket.

Inlet Modifications

 About Flame ArrestorsWe often install aftermarket flame arrestors on many of the high performance PWCs that we construct because they do a good job of meeting a PWCs performance needs.  Sadly, the Yamaha jet-boats have a different set of needs that caused us to retain the stock flame arrestors (with some minor modifications).  We tested at length with aftermarket arrestors on our 270 and 135 Exciters. On both of these machines, the oil injection fittings are mounted in the flame arrestor base, and so we drilled our new aftermarket arrestors to maintain a similar location.  Yamaha injects the oil above the choke plate because it keeps the choke shafts from getting frozen in place (with corrosion).

  Our "arrestor" equipped 135 and 270 would both routinely have heavy oil residue around the filter element, and on the carb support plate.  No matter how often we wiped the residue away, it returned quickly.  The mess was annoying, but not our biggest concern.  This oil that was being spewed about, was oil intended for the engine...that was not getting to the engine.  The variable oil pumps on the 135 and 270 offer (what we consider to be) the minimum amount of oil that these engines should be run on.  For as hard as these engines get run in this application, subtracting "any" amount of oil from that input is a "very" serious situation that is just plain unacceptable.  The inlet manifolds of the 220 were beefy enough to allow us to easily install the injector fittings "below" the carbs, and avoid this problem.  However the inlet manifold castings of 135hp engines in the 135,270, and LS2000 do not safely  accommodate this same modification.

Aftermarket Arrestors Part 2

  ICOMIA (International Council of Marine Industry Associations) is the body that sets the emissions testing standards for all marine crafts (including PWCs and jet-boats).  For reasons too lengthy to explain, the emissions test protocol for PWC engines entails lengthy operation at 75% throttle opening.  As a result, the PWC makers have had Mikuni (the carb maker) construct 44mm  carburetors that have "weakened inlet signal" at this throttle opening.  The result of this weakened signal is a slight "lean spot" that is affected very little by jetting ... it's literally designed into the carbs.  The somewhat restrictive flame arrestors that come on stock engines help to induce slightly higher inlet tract vacuum (aka signal).  With the stock arrestor in place, this built-in lean spot has a very consistent fuel delivery that makes for precise fuel metering at the 75% throttle setting, and so lower emissions during the ICOMIA test.  It bears noting that the increased signal offered by the stock arrestor also improves the "precision" of fuel metering any time that the throttle is maintained at a partial throttle setting for extended periods of time

   When a free-er breathing aftermarket arrestor is installed, the "signal" at this crucial 75% opening becomes weak enough to cause an intermittent lean condition that results in mild detonation (if the throttle is held steadily at 75% throttle for long periods).  Since PWCs are seldom held at one throttle setting for long periods of time (and the sheer weight loads on the engines are "much" lighter than in jet-boats), this varying lean condition is generally not a problem.  On the other hand, jet-boats are very commonly run at great length at one throttle setting (very often being 75% throttle).  . 

   With out a doubt, the stock flame arrestor does slightly harm full throttle performance...but much less than you think.  Since we considered the oil-spewing of aftermarket arrestors unacceptable, we drilled some additional air access holes in the base panel of the stock arrestor case.  These holes made a noticeable improvement in full throttle rpms, and allowed no oil spewing whatsoever.  In later tests, we compared the performance of our modified OEM arrestor against the aftermarket arrestors, and found the difference to be nominal (less than 30 rpm).  At the same time, we felt much more secure about the better "signal" characteristics that this modified stock arrestor would offer at 75%.

  It bears noting that removing the choke plates and shafts from the 44 carbs also reduces signal.  For that reason, we do not remove the chokes from any 44mm jet-boat carbs, and do not recommend it.

  It should be said that the aftermarket arrestors (and the removal of choke plates) are effective mods on most PWCs that run premixed fuel and turn lots higher rpms than these jetboats should be run at.  They are great modifications ... just not great modifications for the needs of the 135, 270, LS200 & XR1800 jetboats.

XR1800 Arrestors -  While we strongly recommend to retain the entire stock flame arrestor case on this model (with no drilling modifications), the rubber tube at the front can be removed to allow better air inlet.  Doing so makes for a nice improvement in overall performance with no apparent down sides.

Exciter 220 Arrestors

  Unlike all the other Yamaha jet-boats, the 220 models use 38mm carburetors with numerous lobes cast into the carb mouths (to increase signal, and thereby offer better metering precision).  The signal of the 38s is so strong that aftermarket arrestors can be applied with good results and no 75% throttle danger zone.  The problem of oil spewing still exists, however (as previously mentioned) the inlet manifold has enough "beef" to safely allow the installation of injector fittings below the carbs (we pull the fittings right out of the stock arrestor plate and use them).  These fittings must be "pressed in" with a slight interference fit to assure no possible air leaks.

About Reed Modifications

Reed Stuffers - All the Yamaha boats (except the Exciter 220) can accept aftermarket reed stuffers.  On PWCs these parts do help low range response, but offer no increase in peak rpm.  From a technical standpoint, they do help to slightly improve inlet tract signal.  During our on-water jet-boat tests, the gains offered by reed stuffers was very hard to define by "feel" or by instruments.  If they have other merits, we could not define them.

Reed Spacers - The claim is that reed spacers increase bottom end power.  Whether that is true or not could not be defined by our instrumented testing.  What we did experience was a minor loss of peak rpm ability that we were unable to recover with carb adjustment.  We do not recommend them.

Aftermarket Reeds - We have tested numerous aftermarket reed petals on the PWC versions of these engines.  While many worked "well" none yielded a "large" definable increase in any part of the rpm range.  That said, we have seen "no" aftermarket reed petals that equal the longevity of the stock oem reed petals, and so we recommend only stock petals and cages for jet-boats.

About Carburetion

 Given the sensitivity issues of signal with the 44mm carbs, we recommend no carb throat modifications at all.  For our kits that use a cylinder-head modification only, no carb jetting changes are required.  The porting of the Sleeper kits does change fuel demands, and so different carb jetting is required.  It bears noting that the stock jetting in the carbs is staggered to help equalize the operating temps of the different cylinders.  The jetting for our Sleeper kits is also staggered.

Long "No Wake" Idling -  The carbs in all the jet-boats comes with the same jetting used in the PWC platforms.  Since the hulls of the PWCs are much narrower than the jet-boats, they more easily list to the left and right.  As an engine idles in a PWC hull, there is a strong vibration at idle (or no wake) speeds.  The narrow hull of a pwc hull actually helps to dampen this vibration by slightly listing left to right in the water at high frequency. 

  The much wider hulls of the jet-boats do not offer this same rotational vibration dampening, and so, they subject the engines (and carbs) to much stronger vibrations at low rpms.  This un-dampened high-frequency vibration can (and does) literally shake the carburetor needle valves off their seats.  When this happens, the jet-boat exhibits an annoying "rich" condition at no-wake speeds.   Worse yet, different adjustment settings on the low-speed metering screws seemed to have very little (if any) effect on this rich condition.  Since we routinely ran our jet-boats through a long no-wake area, this was a problem that had to be solved. 

  After trying many approaches, we solved the problem by switching to heavier 115 gram float springs.  These springs are strong enough to overcome the effects of the heavy vibration, and made our low speed adjuster screw very responsive to different adjustment settings (it now needed to be a bit richer than stock).  With these stronger springs installed, our 135 and 270 test boats could both run smoothly at no-wake speeds forever.  After a long no-wake run, we could plant the throttles and take off with virtually no "loaded-up" fuel that had to be blown out (they both smoked less too).  It bears noting that these springs "do not" work well when installed in a PWC.   For us, this was just another example of how PWCs and jet-boats (using identical engines) have different setup requirements.

About Cylinder Modification

  Cylinder "porting" is a very common, and effective, modification to improve the performance of a 2cycle engine.  At Group K we built the most successful GP 1200 and GPR1200 endurance racing engines in national competition... and cylinder porting was a part of that engine preparation.  However this is another area where the specifications we used for high performance pwcs was not at all what was needed by our jet-boats.

  The LS2000 and Exciters use cylinders that have steel sleeved cylinder walls.  For anyone who has seen one of these off the boat, it becomes immediately clear that there are "huge" port mis-matches between the aluminum cylinder and the steel sleeves.  Correcting this mis-match on the transfer port windows requires very special 90' carbide rotary hand tools, but it can be very beneficial to overall power (particularly bottom end and mid range).  However the biggest mistake many (well meaning) jet-boat owners make is related to the exhaust ports.  For reasons known only to the folks at Yamaha, the steel sleeves mis-match the exhaust port roofs very badly.  However, years of testing with these engines has shown that the stock exhaust port (sleeve) height is ideal for overall performance.  Matching the steel sleeve to the exhaust port casting results in port timings that would be used on a full blown (high-rpm) racing engines.  Making this irreversible modification on your jet-boat cylinders will result in a huge loss of low end power, and a big increase of cylinder operating temperatures.... DO NOT DO IT!!

  When we do our "Sleeper" cylinder porting for the Yamaha jet-boats, we perform all the transfer port matching as well as significant widening and profiling of the exhaust ports.  However in the interest of preserving strong low range power, we do not significantly raise the exhaust ports.  We also offer a "competition finish" option on the porting of our Sleeper kits.  The "Comp" finish porting uses exactly the same specifications as the conventional porting, but the extra time to surface finish the transfer passages and exhaust ports results in slightly better overall performance than the standard finish.  "Comp" finish is for the owner who wants to get every bit of available performance.

About Big Bores

The 1131cc steel-sleeved LS2000 and Exciter engines do not easily accommodate significant increases in bore diameter.  We have prepared .080" over (1184cc) versions, however the added expense of fitting larger sleeves and boring the crankcases give this modification a price tag that is hard to justify for the performance increase that you get.

  Group K makes a 1390cc Big Bore kit for the 1176cc engines in the XR1800.  While these kits were originally developed for the PWC models, they can easily be used on the XR1800 as the.  The cost of doing both engines is considerable, however the increase in power output is HUGE.  This would be a mod for XR1800 owners who have the skill and experience to drive a very high powered boat.  For more info on this kit go to

About Ignitions

  All the 110hp and 135hp engines have built in rev limiters at 7050rpm (+50 rpm).  We do not recommend altering this rev limit or the ignition curves on any of the jet-boats.  Since the stock exhaust systems are not "friendly" to rpms over 7100, there is no reason to try to rev the engines that high.  That said, we do set up all our impeller pitches to run at target peak rpms between 6900-7000.  At the higher end of the ignition curves, timing becomes more retarded, and detonation risk is actually reduced.  Applying a steep prop that has an rpm peak below 6500 can result in much higher operating temps and higher detonation risks because the timing curve is much more advanced at that rpm.... it sounds backwards, but that is the way it works out.

  The XR1800 rev limiters are at 7350 (+50), however the stock exhaust systems are "unfriendly" to rpms beyond 7200.  We recommend target rpms for this model between 7000-7100 rpms for the same reasons as stated above.

About Spark Plugs

 Unlike  the other Yamaha jet-boats, the XR1800 has very high output ignition capable of firing a spark across a .044” plug gap.  This larger plug gap is a big asset to the performance of these engines.  The spark plugs used in the XR1800 are a NGK BR8ES-11.  These plugs are much the same as the more common BR8ES in every way … but one.  These “11” series plugs have a ground electrode with a different radius bend that allows for an accurate .044” gap.  If you try to gap a conventional BR8ES to .044”, the ground post will be bent in a way that does not allow for consistent firing.  If you have an XR1800, use the “11” series plugs gapped to .044”.

About Exhaust Systems - For engines that will be operating with the 7000 rpm limits of the stock ignition systems, the stock exhaust systems work darn good.  The only aftermarket systems built for these engines were the FPP racing triple pipe sets.  These $2500 triple pipe sets allowed for big increases in rpm and horsepower, however they were special built to fit the interior of the PWC hulls , and would likely be a "huge" pain to adapt to a jetboat.

  One small mod that is popular for the "3 into 1" systems is a Jetworx pressure valve kit.  This kit helps the low speed response on PWCs, especially after they have been idled a long time.  The basic premise is a pressure sensitive water valve that keeps water from being injected into the exhaust system until the rpms (and cooling system water pressure) increase to 3000+rpms.  It works great for pwcs that can easily hook up the increase in low speed response.  The only down-side is that the valve can sometimes get clogged with debris.  Given that the jetboats have difficulty responding quickly enough (from a standstill) to benefit, and the possible prospect of clogging, we consider this another mod that has questionable practicality on a jet-boat while it works great on a PWC.

XR1800 exhaust

The “D” Plate – The XR1800 is the only two cycle jet-boat that employs the use of a catalytic converter for emissions reduction purposes.  While the stock exhaust system (with cat-con) works very well on stock and modified crafts, there have been cat-con units that have fractured or broken.  Many owners have been reluctant to simply re-install another cat-con just like the one that broke.  For many owners, the next best option is the installation of a “D” plate.

The European spec GPR1200s do not employ the use of a catalytic converter.  In place of the converter Yamaha has mounted a stainless steel plate with a large “D” shaped hole that exposes roughly half of the exhaust body diameter.  The “D” shaped hole is used because there is a significant amount of back-pressure needed to make the exhaust scavenge correctly.  The engine cannot tell the difference between back-pressure created by the can-con, or back pressure created by the “D” plate, hence there is no performance difference between the two.  Unfortunately, Yamaha has been reluctant to supply these “D” plate in great number to dealers.

Group K offers a duplicate of the European spec Yamaha “D” plate that can take the place of damaged cat-cons.  From an EPA standpoint, removal of the cat-con is legal for machinery used for competition use only.  However it bears noting that there is no external visual difference, and no standardized emissions measurement protocol to police the use of “D” plates.  With the "D" plate installed, the exhaust system temperatures are considerably lower.  Unfortunately, the heat sensor on the pipe body sees this as a malfunction, and will set off a warning beeper.  A plug-in resister is fitted to this heat sensor input to "fool" the sensor into seeing the heat level it wants to see.

About Instrumentation

For all of our jetboat testing, we used GPS to indicate speeds, and digital tachometers to precisely show rpms.  With that, we gain a sense of the accuracy (and inaccuracies) of the stock instrumentation.

  The two instrument styles are the "small face" instruments of the Exciters, and the "large face" instruments of the LS2000 and XR 1800.  We found the large face tach and speedo both to be extremely accurate (within 5% or better).  The "small face tachometers were reasonably accurate up to 5000 rpm (usually reading 50+ rpms too high).  However after 6000 rpms, the small-face tachs routinely indicated 200-300 rpm higher than true reading.  The same applied to the small face speedometer.  Up to 40 mph the small face speedo was only about 5mph high in reading.  However, over 50 mph the stock small-face speedo typically read 10-15 mph high.  With this, we recommend that Exciter owners wanting accurate data should equip their boats with a GPS and a digital tachometer.  The "TinyTach digital tach is an affordable option that provides accurate full time data, and has an hour's money well spent.

About Porpoising - In smooth water and mild wind-chop, the Yamaha boats drive straight and smooth at speed … but rolling wave water is another story.  If the Yamaha boats have one huge shortcoming, it is the chronic nose bouncing, known to pwc riders as “porpoising” (so called because it mimics a porpoise’s jumping movement).  This porpoising takes place anytime the boats encounter sequential rolling waves (like those that come from other passing crafts).  Since the Yamaha boats have no trim system, the porpoising cannot be “trimmed” out (as on larger boats).  There is no foreseeable mechanical “fix” for this problem (bolt-on or otherwise).  However there is a steering “strategy” that can have a huge impact on reducing the problem.

  The hull shape of the Yamaha boats is designed in such a way that a very slight turning of the steering wheel can immediately reduce (or stop) the porpoising action.  When the hull is set slightly to one side (as in a gentle turn) the side face of the hull immediately absorbs the wave action of rolling waves … and the porpoising stops.  The boat can be steered very slightly from side to side (in sweeping turns) to abate porpoising in very rough water conditions.  In situations that do not permit this kind of “constant turning”, merely backing off the throttle will dramatically reduce the porpoising as well.


Exciter 135

Group K fuel/air Separator Kit (electric fuel Pump)                                                           179

Stage 1 Cylinder Head Modification                                   (89 octane)                           180.

Skat Trak “Swirl Impeller (Stg 1 pitch)                                                                                279.

115 gm Float Spring Set (3 springs)                                                                                       9.

 Tiny Tach Digital Tachometer and Hour Meter                                                                    59.


Stage 2 Cylinder Head Modification with cooling upgrade (91 octane)                        195.

Skat Trak “Swirl Impeller (Stg 2 pitch)                                                                                279.

115 gm Float Spring Set (3 springs)                                                                                     9.

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.


Sleeper Engine Modification Kit                                          (91 octane)                          790.

(Includes Recreational Finish Cylinder Porting, carb re-jetting

& Stage 2 head modification with Cooling upgrade

“Competition Finish” OPTION for cylinder porting                                                            120.

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.

Skat Trak Swirl Impeller (Sleeper pitch)                                                                             279.


Exciter 220

Group K fuel/air Separator Kit (electric fuel Pump)                                                           179

Stage 1 Cylinder Head Modification                                   (89 octane)                           180.

Solas Concord Impeller (Stg 1 pitch)                                                                                  279.

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.


Stage 2 Cylinder Head Modification with cooling upgrade (91 octane)     (each)        195.

Solas Concord Impeller (Stg 2 pitch)                                                               (each)        265.

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.


Sleeper Engine Modification Kit                                          (91 octane)     (each)           790.

(Includes Recreational Finish Cylinder Porting, carb re-jetting

& Stage 2 head modification with Cooling upgrade 

“Competition Finish” OPTION for cylinder porting                                                            120.

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.

Solas Concord Impeller (Stg 2 pitch)                                                         (each)              279.


Exciter 270

Group K fuel/air Separator Kit (electric fuel Pump)                                                           179

Stage 1 Cylinder Head Modification                                   (89 octane)                           180.

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.

Solas Concord Impeller (Stg 1 pitch)                                                                                  279.

115 gm Float Spring Set (6 springs)                                                                                     18. 


Stage 2 Cylinder Head Modification with cooling upgrade (91 octane)     (each)        195.

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.

Solas Concord Impeller (Stg 2 pitch)                                                              (each)         279.

115 gm Float Spring Set (6 springs)                                                                                     18.


Sleeper Engine Modification Kit                                          (91 octane)     (each)           790.

(Includes Recreational Finish Cylinder Porting, carb re-jetting

& Stage 2 head modification with Cooling upgrade 

Tiny Tach Digital Tachometer and Hour Meter                                                                    59.

“Competition Finish” OPTION for cylinder porting                                                            120.

Solas Concord Impeller (Stg 2 pitch)                                                         (each)              279.


LS 2000

Group K fuel/air Separator Kit (electric fuel Pump)                                                           179.

Stage 1 Cylinder Head Modification                                   (89 octane)                           180.

Solas Concord Impeller (Stg 1 pitch)                                                                                  279.

115 gm Float Spring Set (6 springs)                                                                                     18


Stage 2 Cylinder Head Modification with cooling upgrade (91 octane)     (each)        195.

Solas Concord Impeller (Stg 2 pitch)                                                              (each)         279.

115 gm Float Spring Set (6 springs)                                                                                     18.


Sleeper Engine Modification Kit                                          (91 octane)       (each)         790.

(Includes Recreational Finish Cylinder Porting, carb re-jetting

& Stage 2 head modification with Cooling upgrade 

“Competition Finish” OPTION for cylinder porting                                                            120.

Solas Concord Impeller (Stg 2 pitch)                                                         (each)              279.



Group K fuel/air Separator Kit (electric fuel Pump)                                                           179.

Stage 1 Cylinder Head Modification                                   (89 octane)                           180.

Solas Concord Impeller (Stg 1 pitch)                                                          (each)             279.

“D” Plate catalytic converter replacement                                                   (each)              59.

Heat Sensor Resistor (needed with D Plate)                                              (each)             21.


Stage 2 Cylinder Head Modification                                   (91 octane)     (each)           195.

Solas Concord Impeller (Stg 2 pitch)                                                           (each)            279.

“D” Plate catalytic converter replacement                                                   (each)              59.

Heat Sensor Resistor (needed with D Plate)                                              (each)             21.



GROUP K 4597 CALLE DEL MEDIA FORT MOHAVE, AZ. 86426  928-763-7600

GETTING THE WORK DONE - Most customers send GROUP K the parts needed for modification via UPS, and then do the engine assembly work themselves.  We also do complete engine and pump assemblies for customers who want a finished unit ready for installation.  Unfortunately, the Yamaha motor exceeds the 150 lb. Limit for UPS and FED EX, so common carriers must be used.

  If you would like to pay additional for 3 day, 2 day, or 1 day return shipment, please specify your preference in a cover letter with your parts.  Be sure to include your return address and day phone information in case we have any questions regarding your order.