Kawasaki 750 – High Performance Mods

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 primary performance problem of the standard 750 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. A secondary performance problem is the lack of peak water speed ability compared to other high performance crafts. Making good increases in the acceleration and peak speed of a 750, without hurting reliability, is not difficult. Reliability of the 750 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 750 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 Hammer 105 kit requires the disabling of the stock rev limiter device, however, the design of our modifications insure that your 750 will not need to be “revving” to make the bulk of it’s power.

In addition to designing kits that are tractable and reliable, we have laid out our kits in a way 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 750 engine has a few inherent design weaknesses. Modifying your 750 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 750 head gasket leaks is high or low spots on the cylinder deck surface. To address this problem, all Group K 750 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 for the 750 engines that offered cooling or sealing abilities that are in any way superior to a properly modified stock head.

AIR LEAKS – The lower end of your 750 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 750 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. (kit includes block offs to test all current Kawasaki, Yamaha & Sea Doo models)

PISTON CLEARANCE – It’s common for a new 750 cylinder to have as much as .006″ of piston to cylinder wall clearance. (set-up clearance is .0035″-.004″ ; wear limit is .008″). Excessive piston clearances (factory set at .005″-.007″) are the primary cause of the piston skirt cracking that many 750 engines have experienced. On any racing or high output recreational packages, we recommend set-up clearances of no more than .004″. This closer clearance virtually eliminates piston skirt cracking. 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 any case, all ported cylinders are honed before return shipment by Group K.

THE “BIG PIN” 750 SERIES – The 1995 and later 750 motors differ greatly from all the previous 750s in several important ways. 1) The wrist pin and big end of the connecting rods have been enlarged for better durability (hence the term “big pin”). This means that the pistons of the early and late engines are not interchangeable. 2) The inlet tracts, reed cages, and carburetors have been downsized slightly to increase inlet tract air speed. This increased airspeed results in much better overall acceleration. 3) The cylinder has much higher exhaust ports than any previous 750s. These larger ports work for these 750s because of the improved low end power offered by the smaller inlet tracts. While some of these parts are interchangeable with earlier 750 models, we don’t recommend it. The SXi cylinder will yield poor results on an Xi and vise versa. The low exhaust port/large inlet tract, and high exhaust port/small inlet tract seem to be matched designs that don’t like being mixed.

TOP END MODIFICATION – All Group K 750 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 750 experience, that may be true. The porting mods used in all of our 750 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. Porting stages are A) Sleeper, B) Hammer 92 octane, & C) Hammer 105 octane.

The port layout in the 750 cylinder is very good from a power output and reliability standpoint. Our porting modification for the 750 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 could sometimes improve peak waterspeeds, however, the strong low end and smooth power delivery were seriously hurt. Furthermore, the increased rpm’s of this porting caused much higher engine temperatures and drastically reduced crankshaft life. We believe the small increase in peak speed isn’t worth the loss of overall reliability.

EXHAUST SYSTEMS – While there are many aftermarket pipes available for the 750’s, we consider only a few of them are a clear cut improvement over the stock pipe. The two best choices for the SX model are the “Factory Pipe Products” LIMITED pipe, and the “Coffman’s Rocket series” pipe (both can use the stock waterbox). For the SS, ST, and Xi models, we recommend the Coffman rocket pipe with waterbox, or the Factory Pipe Products “half pipe”. For the SX, SXi, and XiR we recommend only the Factory Pipe products pipe.

We recommend that a stock waterbox silencer be used on all the Sleeper and Hammer 92 kits. Contrary to rumor, the stock 750 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 750 engine kit) The SX and SXi Hammer 105 should be fitted with a Jetcraft Eng. resonator style muffler. This muffler permits the engine to pull easily into the higher rpm ranges while maintaining tolerable increases in sound level.

INLET SYSTEMS SINGLE CARB – It’s our position that the inlet system that can provide the best inlet tract air speed and fuel mixing, at all throttle positions, is the system that will provide the best overall acceleration and horsepower. While the stock oem Kawasaki dual carb kits can work very well, our testing showed us that a recreational 750 with a well prepared single carb could easily generate as much speed, and acceleration, than most jet pumps can ever deliver. The few aftermarket duals that do offer better performance, do so by consuming “allot” more gasoline. Where riding range is a big consideration, a single carb intake is by far the best choice.

In the SLEEPER kits, the stock 40mm Keihin carb (it’s actually 37.2mm) is bored to a true 40mm. (Note: The 38 mm carbs on the big pin motors are actually 33mm. We make them a true 38.) This throat boring is an extremely cost effective way to increase intake ability without compromising easy starting, smooth idling, easy adjustment or fuel consumption. For customers who want to install a larger single carb, the 42 Keihin carb, used on most single carb Group K Hammer 92 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 750 intake manifold and provides throttle response and acceleration that 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 flame arrestor matches perfectly to the opening of the 42, however the intake manifold must be bored slightly to accommodate the larger 42mm carb throat diameter. NOTE : Boyesen reed stuffers are recommended for all pre 1995 Group K 750 Kawasaki engine kits.

DUAL CARB INLETS – The stock Kawasaki dual Keihins perform as good or better than any aftermarket duals we have seen. Group K offers the same throat boring mod for these as the singles. These bored 40mm(and 38mm) Keihins can easily meet the fuel demands of the most heavily modified 750 racing engines. Our tests showed that perfectly tuned dual 44 or 46 Mikunis offered absolutely no advantage over the bored stockers.

IGNITION – The stock rev limiter on all early 750’s is set at about 7200 rpm. The stock “big pin” 750 ignition’s limiter comes in at about 8000 rpm. We see no need to rev pump gas 750’s beyond these rpms, nor have we seen a power increase by changing the ignition firing “curve”. Given this, we recommend that the stock cdi box be retained for all pump gas 750’s. For customers utilizing the stock ignition flywheel, Group K offers a flywheel lightening modification. While we only lighten it by 1.2 lb, the change is very noticeable because the majority of that weight comes off the outer perimeter of the flywheel where it hurts the inertia the most.

105 octane customers may be interested in a total loss type system. The aftermarket is full of affordable (?) total loss ignition systems that have no limiter. Because of the constant battery maintenance and questionable reliability of these units, we do not recommend them for any recreational or amateur level racing application.

For 105 octane customers who want the absolute best in performance and reliability, we only recommend the Kawasaki 650 flywheel and stator plate used in conjunction with the Dale’s Jetsports “bad attitude” brain box/coil. This conversion offers full charging capabilities along with a much lighter flywheel, increased voltage, and no rev limiter. The increase in acceleration and peak waterspeed is big … very big. The 750 cases do require machining in order to accept the 650 coil plate. The crank cases must be completely empty for this machining process. However for the ultimate high output motor, this trouble-free ignition is easily worth the hassle of a complete engine teardown.

NOTE: Bolt on Ignition Advancers are not recommended for use on any Group K Kits.

COOLING SYSTEM – All of these 750 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 MODIFICATIONS – Pump blueprinting makes no change in peak waterspeed ability. However it does offer much better “hook up” in rough water conditions, and is an important asset to machines that will be run hard in rough water conditions (as race boats are). If your machine will be operated on smooth water only, the blueprinting will offer little improvement for the money spent.

A Skat Trak variable pitch stainless steel impeller will be a mandatory modification for all Group K 750 kits. We recommend a 9′-17′ pitch for the 750 SX, and a 12-20 custom for the Xi & SS (SX engines fitted with a lightweight flywheel will require a 10′-18′). Pending our tests, we will be recommending various pitches of the new Skat Trak “Slim-line” props for the big-pin 750s. Nozzle boring is needed only on the pre ’94 SS models. .

The use of a lightweight driveshaft can improve acceleration ability on racing 750’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 scoop grates are recommended for all kits, however the brand can be crucial. More recent testing done with both the stand up and runabout models showed that the ride plates and scoop grates made by Jet Dynamics consistently offered the best in handling and peak waterspeed abilities.

For recreational machines used primarily in smooth water, cutting the center bar out of the stock scoop grate offers all the additional intake that’s needed. For riders who run their 750 very aggressively in rough water conditions we recommend the Jet Dynamics “top loader type scoop grates. Be forewarned that while these top loader type scoop grates offer excellent rough water “hook up”, they will also scrub 1-2 mph off your smooth water peak speed and cause “eye opening” deceleration whenever you release the throttle.

There are many aftermarket “extended” ride plates that fit the 750 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 ride plates and scoop grates are used, the peak speeds of the Hammer kits will still command your undivided attention. The best stock hulls, even with these bolt on parts, will begin to “seek” at speeds over 50 mph. “Seeking” is the point at which the handlebars no longer solely control the direction that the boat is traveling in. Safe steering past this speed is done with body-english and weight transfer. While various ride plates and scoop grates may reduce high water speed “seeking”, 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 gasoline. 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 no rev limiter (or 400 hours with the stock limiter). On 105 octane race engines, rings should be replaced every 80 operating hours and pistons every 160 hours.