4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Kawasaki JS 550 – Engine Modification Kits

Specs for '91 - '95 Models

This literature will give background information on all of our modifications for the reed valve JS 550 models (’91-’93) as well as some recommendations for the best after market 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 reed engine modification kits are as follows : STD KAWASAKI JS 550 Peak waterspeed (sea level, flat water) ’91-’93 37.8 mph

SLEEPER – Intended for the average recreational rider, this engine kit consists of modification to existing pieces only. The engine compartment of your finished 550 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 – ( flat water) 45.8 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 bolt on parts. The Hammer 92 is as easy starting and controllable as any stocker. Just the same, this is the fastest pump gas compatible 550 reed kit available anywhere. Peak speed – 47.4 mph

HAMMER 105 (octane) – Intended for the serious modified class racer, this kit offers national class power output along with the best in race level reliability. The Hammer 105 is not for the meek or the weak. Peak waterspeed – 49.8 mph

THE GOALS OF MODIFICATION – The goal 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 of the standard JS 550 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 reed 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 insures that your 550 will not need to be “revving” to make the bulk of it’s power.

There are many after market 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. Besides, the high rpm 550 engines used by the sponsored pro racers will typically get a new crankshaft every 25-30 operating hours. No recreational rider can afford that.

DESIGN WEAKNESSES – The 550 reed engine has a few inherent design weaknesses. Modifying your 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 can begin to suffer immediately.

CRANKSHAFT – The stock 550 crankshaft is a reasonably well built unit. Unfortunately, many 550’s will still have the original crankshaft in the engine. For a later model ’93 machine that’s not a problem, however there are many 550’s with one or two hard seasons on the stock crank. Under the loads of stock power output, a two year old crank might last another year. However the added rpms and power loads of even a mild modification can easily be the death blow to a well worn crankshaft. If you’re 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 the crankshaft will 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 through stud holes in the 550 cylinder is very narrow (even 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 stone finished to assure a perfectly flat top sealing surface. It bears noting that no after market cylinder head can resolve the sealing problem. Our tests have consistently shown that most head gasket leak problems usually lie with the cylinder surface and the gasket … seldom the head. Here again, the relatively milder compression ratios used with the Group K kits is an added safeguard against head gasket leaks.

AIR LEAKS – The lower end of your 550 should be totally air tight up to 8 psi. New engines can pass this test without 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 matching of the exhaust manifold. Cylinders and heads are typically prepared as matched pairs 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 port layout in the 550 reed cylinder is outstanding from a power output and reliability standpoint. Our porting modification for the 550 kits has been developed to provide a wide powerband with strong and smooth acceleration at all water speeds. During testing, we found that more radical porting specs could sometimes improve peak waterspeeds, however the strong low end and smooth power delivery were seriously hurt. Furthermore, the increased rpm’s of the radical porting caused much higher operating temperatures and drastically reduced crankshaft bearing life. We believe the tiny increase in peak water speed was not worth the loss of overall reliability.

PISTON CLEARANCE – It’s common for a new JS 550 reed cylinder to have as much as .005″-.007″ of piston to cylinder wall clearance. (set-up clearance is .003″-.0035″ ; wear limit is .008″) On any racing or high output recreational packages we recommend set-up clearances of no more than .004″. Engines intended for moderate recreational use can easily live with .004″-.005″ 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″. 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 after market exhaust components. Both Hammer kits yielded the best low end and mid-range power using the Coffman “Climax” pipe (fits on ’86-’93 550 manifolds). 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 after market companies simply sell their 440 combos for the 550’s …they often work very 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 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. These resonator mufflers, used with the stock rear exit exhaust will easily pass the IJSBA sound level limits.

INTAKE SYSTEM – The reed valves used in the 550 are smaller in size than the reeds commonly used in most after market 550 reed cylinders. It has long been our opinion that the reed cages in the after market 550 cylinders are much larger than they need to be. The smaller cages of the ’91 permit slightly smaller intake tracts that are capable of much higher intake tract air speeds. These high tract speeds are able to move fuel mixture to the engine in a shorter amount of time resulting in much quicker throttle response. Speeding up throttle response in this way has diminished the need for lightweight flywheel total loss ignitions. Granted, these smaller tracts may not be able to pass enough mixture for 52+ mph peak water speeds, but that’s not the performance objective of our kits.

It’s our position that the carburetor that can provide the best fuel mixing, at all throttle positions, is the carb that will provide the best overall acceleration and throttle response. Dual 38 carbs are commonly used on modified race boats because they offer better overall throttle response than a heavily modified 44 Mikuni carb. 38 duals offer better response because the smaller 38 carbs can mix the fuel with the incoming air much more efficiently at partial throttle openings, not because they have greater volume abilities.

The new 42 Keihin carb offers better overall fuel mixing and throttle response than any large throat carb we have tested. This 42mm carb bolts directly onto the stock 550 intake manifold where it works in concert with the small intake runners to provide throttle response and acceleration that few dual carb setups will match. The single 42mm carb has no troublesome linkage, and features the easy “two screw” mixture tuning. (Your stock intake manifold must be bored slightly to accommodate the larger throat diameter of the 42mm carb)

In our tests, the popular K & N flame arrestor 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 the stock flame arrestor. The intake mesh of the stock arrestor is so well guarded 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 stainless steel screen element in any way. The opening on the stock flame arrestor also matches perfectly to the throat opening of the new 42 Kiehin carb.

We have tested several dual carb kits on the reed 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 550 kits.

IGNITION – The stock ’91 550 ignition has a built-in rev limiter that comes in at about 6,500 rpm. By simply disconnecting one wire in the electrical box, the limiter can be eliminated. The unusually heavy flywheel of the stock 550 slows throttle response and acceleration. Our ignition flywheel lightening option offers a very noticeable improvement in throttle response on all three 550 kits with no loss of reliability. The much lighter after market aluminum “charging” replacement flywheels offer a very large increase in throttle response along with an equally large 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 to anyone for applications.

COOLING SYSTEM – All of these 550 kits can operate reliably with the single input cooling system along with 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.

PUMP & IMPELLER – Since all the Group K engine kits are built to produce a wide powerband with strong low end power, the existing 550-style pump is no disadvantage whatsoever, and it works excellent at all waterspeeds. The 550 pump delivers the force of thrust to the hull through the pump case mounting points, not the driveshaft ( like a 440 style pump). 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 550 SX is so flexible, it virtually eliminates the need for shimming the engine and pump altogether.

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 “blueprinting” makes the 550 pump “hook up” as good (if not better) than the best 440 pumps that we have tested. Pump blueprinting offers much better hook-up in rough water 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, blueprinting offers little improvement for the money spent.

A Skat Trak impeller, along with nozzle boring, are mandatory modifications for all Group K 550 kits. The Sleeper kit will give best results with the 15.5′ pitch while both Hammers will work best with a 16.0′. (Hammer 105 riders under 120 lb. may experience better results with a 16.5′ pitch) The use of a lightweight driveshaft can improve acceleration ability on racing 550’s, however it’s often a relatively small performance increase, that comes for a high price. If value for the money spent is a high priority, a light driveshaft should be your last modification.

HANDLING – After market scoop grates are recommended for all kits, however brand is not crucial. The deeper intake scoop of an aftermarket “deep” scoop grate will provide better pump feeding in rough water, while the deeper side rails will improve high speed stability. For riders who run their 550 very aggressively in rough water conditions, we recommend the “top loader” type scoop grates. There are many aftermarket “extended” ride plates available to fit the ’91-’93 JS 550 . Our test riders preferred the stock ride plate, which is significantly extended over all previous ’82-’90 model stock plates. A finned ride plate can give added stability at high speeds.

Regardless of which parts are used, The Hammer will still be somewhat difficult to control at very high speeds, as even the best stock hulls will begin to “seek” at speeds over 44 mph. “Seeking” is the point at which the handlebars no longer control the true direction that the boat is traveling in. Safe steering, beyond this speed, is done with a combination of body english and proper weight transfer. While hull truing may sometimes improve this high speed handling situation, it cannot completely eliminate it.

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, you are welcome to contact us directly for assistance.

RELIABILITY – The Hammer 105 can not and should not be operated at high rpm’s on pump gasolines. In like, the 92 octane kits should never be run on 87 or 89 octane regulars. With a 40:1 mix ratio of Maxima Super M, a full time racing crankshaft can be expected to last approximately 150 hours with the rev limiter disconnected (or 250 hours with the limiter connected). For a racing engine’s maximum power output and minimum wear, piston rings should be replaced every 40 operating hours, and pistons every 80 hours. Recreational engines can easily run an entire season (250-300 operating hours) on one set of pistons and rings.


Group K Price

Includes: cylinder porting, and exhaust manifold matching, cylinder head modification, cooling system upgrade, carb throat boring & circuit upgrade, pump nozzle boring. (SEND head, cylinder, exhaust manifold, carb, and pump exit nozzle)


Sleeper Kit Individual Labor Prices

Group K Price










Engine Labor

Group K Price









Engine Parts

Group K Price





Driveline Labor

Group K Price





Driveline PArts

Group K Price







Additional Options

Group K Price




$105.00* each

*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 !!