4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Kawasaki 650 – High Performance Mods

Engine Modifications for All Models

This literature will give background information on all of our modifications for all the Kawasaki 650 engine equipped models as well as some recommendations for the best aftermarket bolt on parts. We strongly recommend that you read this entire document before deciding which modifications are right for you and the kind of riding you intend to do. Our 650 engine kits are as follows :

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 speed increases average 6 – 8 mph over a stock machine.

HAMMER 92 (Octane) – Intended for the serious recreational rider, this kit shares many of the Sleeper kit mods along with a cost conscious selection of aftermarket bolt on parts. The Hammer 92 is as easy starting and controllable as any stocker, yet it’s the fastest pump gas compatible engine kit available anywhere. Peak speed increases average 9 – 10 mph over a stock machine.

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 speeds average 10 – 11 mph over stock.




Hammer 92

Hammer 105

650 SX

39.2 mph

46.7 mph

50.2 mph

51.4 mph

650 X2

36.8 mph

44.3 mph

46.1 mph

47.8 mph

650 TS

33.7 mph

41.1 mph

43.7 mph

650 SC

33.5 mph

39.8 mph

42.8 mph


27.5 mph

32.8 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 of the standard 650 crafts 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 other crafts. Making good increases in the acceleration and peak speed of a 650, without hurting reliability, is not difficult. Reliability of the 650 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 650 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. Our 650 Hammer kits require the disabling of the stock rev limiter device, however the design of our modifications insure that your 650 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 650 race boats for their sponsored team riders. When recreational riders purchase the same parts and modifications used by these racers, the unfortunate result is often the same unreliable “all top end” engine that the racer has. This is not what most recreational riders want. In fact even among racers, 650 modifieds with lots of bottom end power, and just reasonable top end, are always more desirable.

In addition to designing kits that are tractable and reliable, we have laid out our kits in away that permits easy and affordable upgrading. That is, a customer who has a Sleeper kit can easily upgrade to a Hammer 92 one item at a time (as budgets often require them to do). Upgrading the Hammer 92 to a Hammer 105 can be done with equal ease.

DESIGN WEAKNESSES – The Kawasaki 650 engine has a few inherent design weaknesses. Modifying your 650 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.

HEAD GASKET – All Group K kits are intended to be used with the stock .008″ head gasket. There are many aftermarket alternatives to this gasket, ie. .020″ copper gaskets, .040″ fibre gaskets, and “O” ringed heads. It has been our experience that the primary cause of most 650 head gasket leaks is high or low spots on the cylinder deck surface. To address this problem, all Group K kits include cylinder decking to assure a totally flat deck. While all of the after market gasket replacements may be capable of maintaining a seal, none of them can resolve a leak caused by an irregular cylinder deck surface. Similarly, we have seen no aftermarket head that offered cooling abilities or head gasket sealing that was in any way superior to a properly modified stock head. For customers who want to spend a little extra to get the best, Group K offers a .010″ stainless steel head gasket which has yielded the best results in our testing.

AIR LEAKS – The lower end of your 650 should be totally air tight up to 8 psi. New 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 650 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.

PISTON CLEARANCE – It’s common a new 650 cylinder to have as much as .006″ 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 by Group K.

TOP END MODIFICATION – All Group K 650 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 650 experience, that may be true. The porting mods used in all of our 650 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 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 650 cylinder is outstanding from a power output and reliability standpoint. Our porting modification for the 650 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 is not worth the loss of overall reliability.

EXHAUST SYSTEMS – We recommend that a stock waterbox silencer be used on the Sleeper and Hammer 92. Contrary to rumor, the stock 650 waterboxes yield excellent overall power levels in all pump gas applications. Unlike most after market waterboxes, these stock waterboxes can offer quiet sound levels along with strong power when water is diverted away (water diverters are a part of every Group K 650 engine kit)

The Hammer 105 should be fitted with a rear exit exhaust and 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 together with a rear exit exhaust, will easily pass the IJSBA sound level limits.

INTAKE SYSTEMS – All Group K 650 kits utilize single carb intake systems only. All ’86 – ’90 650cc engines were supplied stock with a “remote fuel pump” 28mm Keihin carb, while all ’91-’93 650 engines were supplied with the newer model “integral pump” 38mm Keihin carb. Group K throat bores both of these carbs for use in their respective SLEEPER kits with the standard intake manifold and flame arrestor case. This throat boring is an extremely easy and cost effective way to increase intake ability without significantly affecting fuel consumption.

HAMMER KIT INTAKES – 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 can 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 42 Keihin carb, used on all Group K Hammer kits, offers better overall fuel mixing and throttle response than any large throat single carb we have tested. This 42mm carb bolts directly onto the stock ’91-’93 650 intake manifold where it works in concert with the small intake runners of the Group K modified ’91-’93 650 manifold to provide throttle response and acceleration that no 44 Mikuni and few dual carb setups will match. The single 42mm Keihin carb has no troublesome linkage, and it features easy “two screw” mixture tuning. Your stock ’91-’93 intake manifold must be bored slightly to accommodate the larger 42mm carb throat diameter. (the early 28mm type manifold will not accommodate 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 Kawasaki ’91-’93 flame arrestor case and element. 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 650 engines. 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 synchronize. That, along with the high price tag, make them a very poor value. Stock ’89-’93 Kaw. reed petals have consistently outlasted and out performed all the aftermarket reeds we’ve tested.

IGNITION – The stock 650 ignitions have a built-in rev limiter that comes in at about 6,200 rpm. It cannot be disabled by simply disconnecting a wire in the electrical box. In past years it was very popular to install a ’86 X2 coil/brain box assembly since they offered higher rpm limits. Due to manufacturing changes in recent years, these ’86 coils have often yielded little to no increase whatsoever in rpm ability . For the money spent, the “Bad Attitude” coil modification available from Dales Jetsports is the best way to eliminate the rev limiter. This modification is an improvement on all others because it increases coil output as well.

Our ignition flywheel lightening option offers a very noticeable improvement in throttle response on all 650 kits, with no loss of reliability. The much lighter aftermarket aluminum charging replacement flywheels will offer an additional increase in throttle response and acceleration. However, the difference in cost between flywheel lightening and the aluminum replacements (about $350) could only be justified by a professional racer. 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 our 650 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 for those systems.

PUMP MODIFICATIONS – Pump blueprinting offers much better “hook up” in rough water conditions and is an important asset to machines that will be run at high speeds in rough water . If your machine will be operated on smooth water only , the blueprinting will offer little improvement for the money spent.

A Skat Trak impeller, along with nozzle boring, are mandatory modifications for all Group K 650 kits. We recommend a 16.0′ or 9′-17′ pitch for all 650 applications except the X2 SLEEPER kit which uses a 15.5′.

The use of a lightweight driveshaft can improve acceleration ability on racing 650’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 in the preparation of your boat, a light driveshaft should be among your last modifications.

HANDLING – Aftermarket grates are recommended for all kits, however the 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 650 very aggressively in rough water conditions we recommend the “top loader type scoop grates. There are many aftermarket “extended” ride plates that fit the SX and X2 models. These longer ride plates can significantly reduce “porpoising” (a big problem on SX models) as well as improve high speed stability.

Regardless of which parts are used, the Hammer will still be somewhat difficult to control at very high speeds. Even the best stock SX hulls will begin to “seek” at speeds over 46 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 body english and proper weight transfer. While hull truing may improve this handling situation, it cannot 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. If you have additional questions about your kit, you are welcome to contact us directly for assistance.

RELIABILITY – The Hammer 105 can not and should not be operated on 92 octane 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 Hammer kit crankshaft can be expected to last approximately 250 hours with the rev limiter disconnected (or 400 hours with the limiter connected). For racing engines, piston rings should be replaced every 80 operating hours and pistons every 160 hours.

Sleeper Kits

Group K Price


Includes: Cylinder Porting, Decking, and Exhaust Manifold Matching, Cylinder Head Modification, Cooling System Upgrade, Carb Throat Boring & Circuit Upgrade, Intake Manifold Modification, Pump Nozzle Boring, and Exhaust Cone Modification. (SEND Head, Cylinder, Exhaust Manifold, Carb, Intake Manififold, Exhaust Cone, and Pump Exit Nozzle)



Includes: Cylinder Porting, Decking, and Exhaust Manifold Matching, Cylinder Head Modification, Cooling System Upgrade, Carb Throat Boring & Circuit Upgrade, Intake Manifold Modification, Pump Nozzle Boring. (SEND Head, Cylinder, Exhaust Manifold, Carb, Intake Manififold, and Pump Exit Nozzle)


Sleeper Kit – Individual Labor

Group K Price









650 “Hammer” Kits – 92 Octane • 105 Octane

Engine Labor

Group K Price





’91 -’93 INTAKE MANIFOLD MODIFICATION (to Accommodate 42 Kiehin Carb)






Engine Parts

Group K Price

’91-“93 MODIFIED INLET MANIFOLD (to Accommodate 42 Kiehin Carb)






Driveline Labor

Group K Price





Driveline Parts

Group K Price








Group K Price





*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 !!