4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Tigershark 650cc Engines – All Models

Group K Modifications

This literature will give background information on our modifications for all the Tigershark 650 engine equipped models 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 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 3 – 4 mph over a stock machine (depending on stock cdi rev limit).

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 additional mods and some 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 for the 650 Tigershark’s. Peak speed increases average 5 – 6 mph over a stock machine.

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 Tigershark 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. The rev limiters in the stock ignitions assure that this will not happen. NOTE: Group K does not recommend “eliminating” the stock 7100 rpm limiter. Turning rpms in excess of 7100 rpm will can result in operating temperature increases that will mandate the need for expensive high octane race fuel. All of our testing with 650 engines has shown that competitive and reliable horsepower levels can be had without spinning the engine into the “twilight zone” rpm range.

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).

DESIGN WEAKNESSES – The Tigershark 650’s have 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, i.e. .020″ copper gaskets, .040″ fiber 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 surfacing 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 and surfaced stock head.

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.

LOW SPEED PUMP CAVITATION – The low speed cavitation experienced by 650 owners is more related to pump design than incorrect tolerances. This means that aftermarket pump modifications will be able to diminish the problem, but never completely eliminate it. Reducing this cavitation is important for reliability as well as performance. When a boat operates in a mode of non stop cavitation, the coolant delivered from the pump to the motor is more a foam than a liquid. The reduced cooling system efficiency causes much higher operating temperatures that can eventually result in engine problems. The constant cavitation also erodes the base of the impeller blades in a way that looks very much like “termite” erosion. The eventual result of this damage is impeller blade failure. These risks apply to stock machines as well as Sleepers.

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 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.

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 has very good potential 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. The “rotated axis” design of the cylinders and pistons looks odd and unconventional at first. However, this design represents no technical handicaps or obstacles for the construction of a reliable and powerful “pump gas compatible” engine. We believe this would not necessarily be the case for a “high rpm – high compression” 105 octane racing engine arrangement. For this reason, Group K does not offer a 105 octane racing engine package. The additional maintenance costs of such an engine would likely not justify the relatively small increases in performance.

PISTON CLEARANCE – It’s common for a relatively 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 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 .010″ and .020″.

While Group K questions the performance merits of the port window in the 650’s rear piston skirt, there is no questioning the long term weakness they represent in a high out put engine. For engines that will be run constantly at maximum rpm and maximum load, Group K recommends the fitting of Wiseco pistons. The 76.0mm standard sized Wisecos will slip right into the earlier 643cc engines. However boring will be required to fit them into the 75.6 mm bores of the later 639cc models. In any case, all cylinders are honed before return shipment by Group K.

EXHAUST SYSTEMS – The 650 Tigershark’s come with 2 different types of exhaust pipes. The “Montego” stainless steel bodied pipes, and the so called higher performance cast aluminum “Daytona” style pipes. On Sleeper kit equipped machines, the difference in performance between these two pipes is negligible. Both pipes, on a modified machine, will offer good low end power as well as bump the stock 7100 rpm rev limiter.

The same waterbox muffler is used in all 650 models. We recommend no aftermarket replacement since it performs as well as, if not better than, anything we tested.

INTAKE SYSTEMS – Here again, there are three different systems whose performance differences are negligible on the Sleeper equipped 650’s. The single 44mm Mikuni, single 46mm Mikuni, and dual 38mm Mikuni carb arrangements are all capable of strong overall acceleration and bumping the 7100 rpm limiters. For best results with the single carb models, Group K does recommend replacing the choke plate with a primer. The Sleeper 650’s always started instantly under normal conditions. The primer is only needed for the first start up on very cold mornings.

The only modification Group K makes to the inlet system is the machining of the inlet manifold. This modification offers a noticeable improvement in overall acceleration and throttle response.

The stock flame arrestors on all models, with minor modifications, offered the same performance increases as the more expensive bolt on type aftermarket arrestors. For customers who wish to buy aftermarket anyway, we recommend the Jetinetics “TFS tall” flame arrestor.

We strongly recommend the use of the stock reed petals and reed cages. The stock reeds and cages out performed any aftermarket reeds we have tested. Some aftermarket reeds can offer better power for 20 – 30 operating minutes. However after that amount of time the material “looses its memory” or begins to chip. As this happens, the performance offered by these weakened aftermarket reeds becomes less than stock.

IGNITION – All the 650’s are equipped with a Mitsubishi ignition system that is virtually identical to the system found on the 750 Kawasaki models. The 650 models come stock with either a 6800 rpm or 7100 rpm rev limiter (refer to your owners manual for info on your particular model). The 6800 limiter permits 48 – 49 mph speeds while the 7100 limiter permits 51 – 52 mph. The 7100 rpm brain box is available from Arctco, and bolts right in place of the 6800 unit.

While the ignition advancers and aluminum flywheels made for 750 Kawasaki’s will bolt onto your 650, Group K does not recommend them. The advancer plates offer excessive spark advance that can cause serious over heating on the Tigershark 650’s. Some 750 Kawasaki aluminum “charging” ignition flywheels have caused stator plate failures on various models. It is unclear whether these failures are a result of component incompatibility or varying magnetic fields. Whatever the case, the risk of failing an expensive stator plate is not worth the potential gains. To this end, Group K does offer lightening of the stock ignition flywheel. While only .7 lbs. is removed, the increased acceleration is very noticeable because we remove the material from the outer perimeter of the flywheel where it hurts the inertia the most. We feel that this is the most weight that can be removed without compromising the long term life of the flywheel.

COOLING SYSTEM – The stock 650 cooling systems are very effective and efficient. All our 650 Sleeper kits use the stock cooling systems “as is”.

PUMP MODIFICATIONS – As of this writing, Solas and Nu-Jet are both preparing to release aftermarket impellers for the 650s. We will test them as soon as production units are available. In the meantime, the stock stainless steel prop used in the high performance models works very well.

For lightweight riders (under 140 lb), a re-pitched stock prop on the Sleeper kit can offer a good gain in hook up and peak speed. This prop re-pitching is available from TJ’s Performance in Wichita Kansas. NOTE: When replacing an aluminum prop with a stainless one, the aluminum impeller housing must also be converted to the stainless steel housing (available from Arctco).

HANDLING – By far the best ride plates and scoop grates for the Tigershark 650s are from Abaco of Daytona Beach Fla. The ride plates offer improved handling as well as increased water speeds. The scoop grates offer improved “hook up” along with only nominal losses in peak water speed.

For customers who want a quick and easy way to improve hook up, cutting the center bar out of the stock scoop grate is the next best alternative.

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’re welcome to contact us directly for assistance.

FUTURE DEVELOPMENTS – Additional 650 Tigershark testing is being done by Group K through the fall of 1995. The primary focus of this testing is to develop a 92 octane engine format centered around a new 7500 rpm cdi ignition box. Along with this, will be development of modifications for the existing stock cast aluminum pipe. Testing will also be done with modifications that can further reduce the low speed cavitation…stay tuned.

Package / Upgrade / Parts

Group K Price


  • Cylinder Porting

  • Decking

  • Cylinder Head Modification and Intake Manifold Matching

  • (SEND: Head, Cylinder & Intake Manifold)


Individual Prices













7100 rpm CDI BOX






T.J.’s IMPELLER (steeper pitch)


Additional Options





*prices subject to change based on manufacturer’s current 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 !!