4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Kawasaki JS 550 – High Performance Mods

Specs for '82 - '90 Models

This literature will give background information on all of our modifications for the piston port JS 550 models (’82-’90) as well as some recommendations for the best aftermarket bolt on parts. We strongly recommend that you read this entire pamphlet before deciding which modifications are right for you and the kind of riding you intend to do. Our 550 engine modification kits are as follows: STD KAWASAKI JS 550 Peak waterspeed (sea level, flat water) ’82-’85 35.2 mph ; ’86-’90 36.5 mph

SLEEPER – Intended for the average recreational rider, this engine kit consists of modification to existing pieces only. The engine compartment of your finished Sleeper will look 100% cosmetically stock. The Sleeper will start and idle like a stocker, have the sound level of a stocker, and run on 92 octane pump gas. Peak waterspeed (sea level, flat water)’82-’85 43.8 mph; ’86-’90 45.1 mph

HAMMER 92 (octane) – Intended for the serious recreational rider, this 550 kit shares many of the Sleeper kit mods along with a cost conscious selection of aftermarket bolt on parts. The completed Hammer 92 is as easy starting and controllable as any stocker. Just the same, this is the fastest 92 octane pump gas compatible 550 piston port kit available anywhere. Peak speed – (sea level, flat water) 38 carb 45.6 mph ; 44 carb 47.2 mph

HAMMER 105 (octane) – Intended for the serious modified class racer, this kit offers national class competitive power output along with the best in race level reliability. This Hammer kit requires 105+ octane fuel at all times. Peak waterspeed – (sea level, flat water) 48.8 mph

THE GOALS OF MODIFICATION –  The goals of every GROUP K modification is to offer the maximum possible overall power increase while making the minimum possible loss of reliability and power band range. The biggest performance problem the standard JS 550’s is the lack of low speed acceleration. Strong low speed power is needed to permit smooth controlled turns, as well as getting heavier riders up on plane. The next biggest performance problem is the lack of peak water speed ability compared to all other crafts. Making good increases in the acceleration and peak speed of a 550, without hurting reliability, is not difficult. Reliability of the 550 engine is seriously affected only when the engine is forced to turn excessive rpm’s or when compression ratios are set too high. All of our testing with 550 piston port engines has shown that competitive and reliable horsepower levels can be had without excessive compression, or spinning the engine into the “twilight zone” rpm range.

All our 550 kits require the disabling of the stock rev limiter device, however the design of our modifications insure that your 550 will not need to be “revving” to make the bulk of it’s power.

There are many aftermarket companies that build high rpm and high compression 550 race boats for their sponsored team riders. When recreational riders purchase the same parts and modifications used by these racers, the unfortunate result is the same unreliable “all top end” engine that the racer has. This is not what any recreational rider wants. In fact even among racers, 550 modifieds with lots of bottom end power, and just reasonable top end, are always more desirable. The high rpm engines used by the top pro riders will typically get a new crankshaft every 25-30 operating hours. Few recreational riders can afford that.

DESIGN WEAKNESSES  – The 550 engine has several inherent design weaknesses. Modifying the piston port 550 with a keen awareness of these weaknesses is what permits the construction of an effective yet reliable modification package. If these weaknesses are ignored, engine life will begin to suffer immediately.

CRANKSHAFT – The stock 550 crankshaft is actually a reasonably well built unit. Unfortunately, many 550’s will still have the original crankshaft in the engine. For a later model ’90 machine that’s not a problem, however there are many 550’s with four or more seasons on the stock crank. Under the loads of stock power output, that four year old crank might last another two years. However the added rpms and power loads of even a mild modification can easily be the death blow to a well worn four season crankshaft. If your considering modifications, and you doubt the integrity of the crankshaft … replace it. We recommend that you replace it with a 440 crank (known as a 3011 crank because those are the last four digits of the Kawasaki part number). Strangely enough, the 3011 crank is dimensionally identical to the 550 crank, but made from much better materials. Truing and welding either crankshaft would only be necessary if you intend to use the machine for racing or very hard recreational riding.

HEAD GASKET SEALING – The amount of head gasket sealing area between the cylinder bores and the thru stud holes in the 550 piston port cylinder is very narrow (much narrower than the 440). Sealing on an area that narrow is easy on the low compression/low operating temperature stock engine. However when operating temperatures, and compression ratios are increased, the job of maintaining a long term seal is much tougher. If the cylinder is bored to an oversize, that critical area becomes narrower yet. All GROUP K ported cylinders are decked and then stone finished to assure a perfectly flat sealing surface. Along with that, all Hammer kits and over bored Sleeper kits are recommended to be assembled with the durable and reusable .040″ solid copper head gasket. It bears noting that no aftermarket cylinder head can resolve the sealing problem. Our tests have consistently shown that the problem lies with the cylinder surface and the gasket …not the head.

AIR LEAKS – The lower end of your 550 should be totally air tight up to 8 psi. Newer engines pass this test with no problems. However engines with deteriorated crank seals or brittle intake gaskets can allow outside air to leak into the lower end during operation. This “air leak” can make the fuel mixture lean enough at high rpms to cause a piston seizure. Air leaks, not engine modifications, are by far the number one cause of 550 piston seizures. GROUP K offers a simple to use pressure test kit that allows you to quickly test for any air leaks that your engine may have.

TOP END MODIFICATION – All GROUP K 550 kits include cylinder porting. In the past, cylinder porting has received an undeserved reputation as a modification that causes a loss of reliability and engine life. If the porting is performed by someone with very little 550 experience, that may be true. The porting mods used in all our 550 kits is the result of many months of performance and wear evaluation testing. We confidently claim that GROUP K cylinder porting will yield a wider power band and longer piston life than any other porting modification available anywhere.

All GROUP K porting includes cylinder decking and matching of both manifolds. Cylinders and heads are typically prepared as matched pairs only to assure that you have the proper squish clearance and compression ratio for the quality of gasoline that you intend to use. These cylinder and head specifications play the key role in determining the amount of overall power that the engine can make as well as the quality of fuel that you must use. The Hammer 105 can not and should not operated at high rpm’s on pump gasolines. In like, the 92 octane kits should never be run on 87 or 89 octane regulars.

PISTON CLEARANCE – It’s common a new 550 iron cylinder to have as much as .008″ of piston to cylinder wall clearance. (set-up clearance is .0045″-.005″ ; wear limit is .009″) On any racing or high output recreational packages we recommend set-up clearances of no more than .006″. Engines intended for moderate recreational use can easily live with .006″-.007″ set-up. Piston clearance inspection is available on request, and cylinder boring (using pistons furnished by you or us) is available in oversizes of .020″and .040″. One trick that is commonly used to tighten up a loose bore in the piston port 550 is the installation of ’91 550 reed pistons. These ’91 dual ring pistons run consistently .004″ larger than the single ring “piston port” type pistons. It is simple and inexpensive for us to size your worn iron cylinder to accept a pair of tight fitting ’91 type pistons in std., 1st and 2nd oversizes. In any case, all ported cylinders are honed before return shipment.

EXHAUST SYSTEMS – While the Sleeper kit utilizes the complete stock exhaust system, the 550 Hammer kits are intended to be used with aftermarket exhaust components. Both Hammer kits yielded the best low end and mid-range power using the Coffman “Climax” pipe (fits on ’86-’93 manifolds only). If you intend to use a 440 style manifold/pipe combo, we recommend only the Coffman “Classic” pipe and manifold. The Coffman systems have been designed specifically for the 550. As a result “they work” excellent. Many other aftermarket companies simply sell their 440 combos for the piston port 550’s …they usually work poorly.

We recommend that a stock waterbox silencer be used on the Sleeper and Hammer 92. Contrary to rumor, the stock waterbox yields very good overall power levels in all pump gas applications. The Hammer 105 should be fitted with a rear exit exhaust used together with the Jetcraft Eng. dual resonator mufflers. These mufflers permit the engine to pull easily into the higher rpm ranges while maintaining tolerable increases in sound level. The resonator mufflers, used with a side exhaust exit will not pass the IJSBA or U.S. Coast Guard sound level limits. The Jetcraft Eng. muffler/rear exit tube gave our 550 test boats the best overall performance and best sound levels when resonator mufflers where used in the nose of the boat. With many rear exit systems we experienced terrific “deep water re-start” problems due to the amount of water that backed up in the exit tube. The Jetcraft rear exit offered the best deep water starts of any system we tested. For recreational use, the Fischer brand waterbox also worked well.

INTAKE SYSTEMS – The standard 44 Mikuni carb is an excellent carburetion choice for all modification levels of the 550. Internal circuit upgrades are mandatory for all the 550 kits. Since the choke plate screws so commonly loosen and fall down into the engine, replacing the choke mechanism with a primer system is recommended for all 550 Hammer’s. Carb throat boring the 44 has consistently proven to result in poor starting and poor low speed response…we do not offer it or recommend it.

We have tested several dual carb kits on the piston port 550’s. Surprisingly they offered only slight increases in overall performance (about 1 mph). All the duals we tested proved to be very difficult to tune and keep synchronized. That, along with the high price tags, made them a very poor value. As a result we recommend dual carbs to no one, and we do not use them on any of our piston port 550 kits.

For the Sleeper, we provide a double mesh stainless steel screen to replace the pumice stone element in the stock flame arrestor case. In our tests, the popular K & N replacements performed well as long as they were perfectly dry. However when they became even a little wet, they offered very restricted air intake ability until they were removed and allowed to dry out. For our Hammer kits we recommend only the Westcoast velocity stack flame arrestor replacement. The intake mesh of this element is so high in the engine compartment that it is exposed to very little water spray. Whatever spray it may be exposed to will not clog the all screen element in any way. Boring the Westcoast stack to perfectly match the mouth of the 44 carb is available as an option.

IGNITION – The stock 550 ignition has no built in rev limiter, however it does have an unusually heavy flywheel that slows throttle response and acceleration. Our ignition flywheel lightening option offers a very noticeable improvement in throttle response on all three 440 kits with no loss of reliability whatsoever. The much lighter aftermarket aluminum charging replacement flywheels offer an incredible increase in throttle response along with an equally incredible price tag. The price of these aluminum “charging” flywheels would easily be worth the performance for a racer with a Hammer 105. However for the recreational rider, we recommend the lightening option. The reliability of the total loss ignitions we’ve tested has been so poor that we do not recommend them.

COOLING SYSTEM – All of these 550 kits can operate reliably with the single input cooling system and our cooling system upgrade. For customers who prefer to prepare their machines with a dual or oversized input systems, we can also prepare the special exit plumbing needed to make those systems work.

DRIVELINE – The force of impeller thrust of the 550 pump is delivered to the pump mounting points at the back of the hull , not to the driveline bearing holder mounted on the back wall of the engine compartment (like the 440). As a result, no bulkhead bearing support is needed (like on all 440’s)

The use of a lightweight driveshaft can noticeably improve acceleration ability on racing 550’s, however it’s often a relatively small performance increase that comes for a very high price. If value for the money spent is a high priority for you, a light driveshaft should be your last modification.

PUMP & IMPELLER – In the past, most heavily modified 550’s were converted to use a 440 style pump with an expensive stainless steel vane insert. The 440 type impeller was usually mounted on an expensive aluminum or titanium driveshaft that’s held by the bearing housing and an aftermarket bulkhead support. The 440 pumps were used because they can process more water at higher rpm’s than the 550 pump can. These high rpm’s are needed to generate the 50+ speeds on some pro mod racers. The 440 impeller delivers the entire load of it’s thrust to the drive shaft holder that is mounted on the rear engine compartment wall. This load is so great that the holder needs an additional support mounted to the floor of the engine compartment. The 550 style pump delivers the force of thrust to the hull through the pump case mounting points, not the driveshaft. This safely permits the use of a lighter driveshaft and eliminates the need for a bulkhead bearing support. In fact, the free floating bulkhead bearing system in the 1990 model 550 sx is so flexible, it virtually eliminates the need for shimming the engine and pump altogether. Another disadvantage of the 440 arrangement is it requires that the pump, driveshaft holder, and engine assembly be perfectly shimmed in the fiberglass hull so that they are all in alignment. Even if this shimming is perfectly executed, the impeller will regularly “crash” the rear hull mounted pump case due to hull flexing and engine torquing that takes place during cornering . Impeller replacement, with 440 pumps becomes a common maintenance procedure.

All JS 550 pumps have a “pump mounted” impeller. This system permits much closer working tolerances between the impeller and the pump housing without risk of collision. This closer fit offers much better thrust at low engine speeds and results in more controllable acceleration off the corners. Our pump blue printing makes the 550 pump “hook up” as good (or better) than the best 440 pumps that we have tested. Pump blueprinting offers much better “hook up” in rough water conditions and is an important asset to machines that will be run hard in rough water (as race boats are). If your machine will be operated on only smooth water, the blueprinting will offer little improvement for the money spent.

All early style 550 pumpcases were manufactured with no stainless steel impeller housing liner. These early housings were adequate for stock machines, but fall short badly for high output use. If you have an early style impeller housing, it should be replaced with a new version 550 housing that has the stainless liner. A 15′.0 or 15.5′ Skat Trak impeller and, along with nozzle boring, are mandatory modifications for all Group K 550 kits. Riders under 120 lb. may experience better results with a 16.0′ pitch on the Hammer 105 only. No piston port 550 on earth can pull a 16.5′ or steeper prop.

HANDLING – For all 550 models, a slightly longer aftermarket ride plate is a big handling asset. The longer plate reduces “porpoising” (the nose of the jetski bouncing uncontrollably) and improves high speed stability. An aftermarket scoop grate will help to maintain a better supply of water at the front side of the pump. Aftermarket grates are recommended for all kits, however brand is not crucial. For riders who run their 550 very aggressively in rough water conditions we recommend the “top loader type scoop grates.

ASSEMBLY INFORMATION – All GROUP K kits are accompanied by a step by step instruction pamphlet that outlines the assembly, break in, fine tuning, and maintenance of your kit. For any further questions you may have regarding your Sleeper or Hammer kit, your are welcome to contact us directly for assistance.


Group K Price

Includes: cylinder porting, decking, and manifold matching, cylinder head modification, cooling system upgrade, carb circuit upgrade, flame arrestor modification, pump nozzle boring. (SEND head, cylinder, manifolds – intake and exhaust, carb, flame arrestor, and pump exit nozzle)


Sleeper Kit Individual Labor Prices

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Engine Labor

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Engine Parts

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Driveline Labor

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Driveline PArts

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SKAT TRAK IMPELLER – SLPR 15.0′ • HAMMER 15.5′ (’82-’89)










Additional Options

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*prices subject to change based on manufactures pricing


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. The 150-lb. UPS weight limit makes engine shipping practical and affordable. NOTE: Group K will bill an additional $25.00 handling charge for complete engine assemblies. All orders prepaid with a cashiers check or money order will be returned freight free via ups ground service anywhere in the continental United States. All other orders will be billed to a visa/master card or sent freight collect cod cash. 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. PACK YOUR PARTS CAREFULLY !!