4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Yamaha 633-701 – Recreational Engine Mods

Group K Modifications

This literature will give background information on all of our modifications for all the Yamaha 633cc & 701cc engine equipped models as well as some recommendations for the best 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 Yamaha engine kits are as follows:

Sleeper – Intended for the average recreational rider, this relatively inexpensive 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.

Hammer 96 (Octane) – Intended for the serious recreational rider, this kit shares many of the Sleeper kit mods along with a cost conscious selection of bolt on parts. The Hammer 96 is as easy starting and reliable as any stocker.

Single Carb Models




685 Sleeper

Hammer 96

633 LX

36.8 mph

42.7 mph

44.6 mph

633 VXR

35.5 mph

42.4 mph

45.6 mph

46.8 mph

633 Wave Runner 3

34.7 mph

42.1 mph

44.2 mph

45.2 mph

633 Super Jet

38.2 mph

45.3 mph

47.6 mph

48.2 mph

701 Super Jet

44.1 mph

48.4 mph

52.1 mph

701 VXR

44.6 mph

48.7 mph

50.3 mph

701 Wave Blaster

43.8 mph

47.5 mph

51.6 mph

Dual Carb Models




Hammer 96

Big Bore Hammer

701 Super Jet

46.1 mph

48.4 mph

52.1 mph

55.8 mph

701 Wave Blaster

44.8 mph

48.5 mph

51.6 mph

55.3 mph

701 Wave Raider ’94

53.6 mph

57.7 mph

60.3 mph

61.8 mph

701 Wave Raider ’95-’96

50.4 mph

53.8 mph

56.1 mph

58.2 mph

ABOUT HIGH PERFORMANCE PWC’s AND THE NEW REFORMULATED FUELS – For the last 7 years, Group K has offered two lines of pump gas engine modification kits known as “Sleeper Kits” and “92 octane Hammer Kits”. The Sleeper kits are an affordable group of modifications to stock components only. The 92 Hammer kits were designed to be the highest horsepower combination, of modifications and bolt on parts, that could run reliably on 92 octane pump gas. All our Sleeper kits are still safe to run on reformulated 92 octane. However the high performance “unfriendliness” of the new reformulated fuels is so profound that we have had to increase the octane mandate for the Hammer kits to a minimum of 96 octane.

Part of the reason for the increased octane need is the nature of the modern personal watercraft themselves. During the days of stand up pwc’s, it was rare for anyone to ride “full throttle-full rpm” for extended periods of time. However, the later model (sit down) machines handle so well that extended “full rpm” operation has become commonplace. We test our kits with this very heavy use in mind. Initially we were able to avoid the 96 octane mandate by using stock rev limiters and milder compression. However, the progressively increasing output of new machines and the decreased quality of 92 octane pump gas has brought about this unavoidable situation.

There have certainly been many isolated situations where owners have “gotten away” with running pump gas in machines that we might consider as 96 or 110 octane formats…good for them. However these exceptional situations will become more and more rare. We have no idea how the other aftermarket performance shops will be dealing with this issue, but we do know they cannot ignore it.

Octane Options – Not many folks want to drive a long distance to spend $5.00 a gallon on race fuel for their recreational pwc. However anyone that has paid an expensive bill for the repair of a seized piston has considered it. The repair cost of a seized piston is the financial basis we will use to look at the cost effectiveness of various octane options. The average repair bill for a seized piston is about $350 for a twin cylinder pwc.

By far the most cost effective octane option is a 50/50 mix of avgas. A gallon of 100-octane avgas costs about 50 cents more per gallon than 92 octane unleaded. Based on a 10 gallon tank of gas, you could buy 140 tanks full of 50/50 100 octane avgas/92 pump gas for the same money it costs to repair one seized piston.

Another option is a 25% mix of 110-octane race gas with 92 pump. With race gas at $5 per gallon, you could buy 28 tanks of this mix to equal the repair of a seized piston.

Lots of owners ride in areas where they need to buy fuel at outlets on the water. These folks are more interested in octane “boosters” that they can carry along. Unfortunately no octane booster can turn 92 octane into 96. At best, 3 oz of booster per gallon can normally make 87 into 92, or 92 into 93.5 – 94 (the increase offered by booster becomes less as base octane gets higher). Using more than 3oz per gallon offers no further increases. In addition, this small increase comes at an expensive $1 per gallon (the 25% mix of race gas costs $1.25 per gallon).

Design Weaknesses – The Yamaha engines have a few inherent design weaknesses. Modifying your Yamaha 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 and reliability can begin to suffer immediately.

Head Gasket – All Group K kits are intended to be used with the stock .040″ head gasket. There are many aftermarket alternatives to this gasket, i.e. .020″ copper gaskets, and “O” ringed heads, etc. It has been our experience that the primary cause of most head gasket leaks is 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.

Air Leaks – The lower end of your engine should be totally air tight up to 8 psi. New engines pass this test with no problems. However engines with older 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 piston seizures on the Yamaha twin motors.

Driver Coupler – The stock drive couplers and rubber cushion, mounted at the back of the motor, can work well on a stock machine. However these parts may have difficulty standing up to the added torque loads of the Hammer 96 engines (particularly in the Wave Raider hulls). As a safety measure, we recommend a set of “billet” drive couplers for modified machines that will frequently be run at full rpm and full load.

Top End Modification – All Group K 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 Yamaha experience, that may be true. The porting mods used in all of our kits are 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 modified 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 matched 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 of the stock 633cc Yamaha cylinders is by far the mildest in the entire watercraft industry. For this reason, the 633cc engines gain huge overall power margins from cylinder porting. The 701cc engines have a more advanced standard port layout, however the power gains made by cylinder porting are still very significant. In both cases, the most advanced port layouts used in our kits have a negligible affect on long term piston or ring life.

Piston Clearance – It’s common for new 701 cylinders to have as much as .005″ of piston to cylinder wall clearance. (set-up clearance is .003″ – .0035″ ; wear limit is .007″). 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 all oversizes. Since Yamaha only offers only two oversize pistons, boring beyond second over will mandate the use of Wiseco pistons. In any case, all ported cylinders are honed before return shipment.

Big Bore Conversions for 633’s – The availability of the newer 701cc top end has moved many 633cc owners to consider a big bore upgrade. There are two common ways to accomplish the 633 big bore. They are as follows:

Installing the 701cc Top End “Spending with the Big Boys” – The 633 cases need to be bored slightly in order to accommodate the larger sleeves of the 701cc cylinder. With this done, the entire 701 top end will bolt directly on. From a performance stand point, the stock 701 top end will run much stronger than a stock 633 top end. However the stock 701 conversion (@ $800 parts & labor) is no more reliable, and very questionably faster, than a 633 Sleeper kit (at $499). A fully modified 701 top end ($499 added to the $800) is much faster overall than the 633 Sleeper, and just as reliable. The modified 701 top ends generate “very” strong middle and high range power. Since the 701 cylinder has more available port area, and thereby more high output potential, a 701 cylinder on 633 cases (at about $1300) is a popular (and competitive) combination used by many riders.

Big Boring the 633 Top End to 701cc “An Affordable Alternative” – The Group K 633 Big Bore Kit offers performance very close to that of the 701 Sleeper. Simply boring the 633 cylinder to 701 works very poorly because the 633 port windows are far to small to effective feed the 701 bores. For this reason, cylinder porting, head modification, carb upgrading, and cooling system upgrading are absolutely mandatory parts of this big bore conversion. With all this done, your 633 big bore will blow off stock 701s, and be within a few percent of modified 701’s. We consider the 633 Big Bore (in both Sleeper and 96 Hammer formats) to be the most reliable of the 701cc high performance setups we construct. The total cost difference between a 633 Big Bore Hammer 96 and a 701cc Hammer 96 is about $600. The difference in overall power is about 1.5 mph of peak speed ability. If you’re a racer, that’s everything. If your not a racer, 1.5 mph is questionably worth an extra $600.

Big Bore Conversions for the 701s – 753cc-781cc Big Bore Kits – Group K offers two different Big Bore modifications for the 701 engines, based on the cylinder that you have. The first, and most affordable, is the Raider Cylinder Bore Only. The 701 Raider cylinder (which bolts onto SJ/Blaster cases with no case boring) can be bored to 753cc using the existing stock sleeves. While the base sleeve areas do become somewhat thinner, the are easily strong enough for high output long term use. Boring this cylinder leaves the port windows very undersized for a 753cc. That’s why cylinder porting will be a mandatory part of all Group K big bore kits.

We will fit 84mm bores to Wiseco pistons because they typically run .002″ – .003″ under the size of the stock Yamaha 84mm pistons. In the event that this original bore becomes slightly worn or scored, the Group K big bore cylinder can be honed slightly to a perfect clearance fit on the slightly larger Yamaha piston. After that, the .010″ and .020″ Yamaha piston can still be used as additional overbore sizes. This amount of over bore allowance would easily last most owners a couple of seasons.

The second type of big bore is the Sleeve and Bore, which must be done on the SJ/Blaster (61x coded) 701 cylinders. These cylinders do not have enough base sleeve thickness to safely accommodate an 84mm piston (753cc). For these cylinders, Group K installs a thicker wall sleeve (made to our specifications) that can safely accommodate pistons up to 85.5mm (781cc). Top case boring is required to accommodate these larger sleeves. This same “thicker wall” sleeve installation will be available for customers who desire to bore their Raider cylinders to the 85.5mm diameter (boring the Raider cases will be necessary).

Case Porting – Case porting by itself doesn’t offer a giant overall power increase on the 701 engines. However, this is a modification that can make a noticeable improvement in overall power. This increase is very desirable to many owners because it comes without any loss of reliability at all. The case porting is an “all gain – no lose” modification that should be done on any Big Bore Hammer. (note: cylinder is required with top crankcase half to do case porting)

Exhaust Systems – We recommend that a stock waterbox silencer be used on all the Sleeper and Hammer kits. Contrary to common belief, the stock Yamaha waterboxes yield excellent overall power levels in all high output applications. Unlike most after market waterboxes, the stock waterboxes can offer quiet sound levels along with strong power even when water is diverted away (water diverters are a part of every Group K engine kit). Even the 701cc Hammer 96 runs best with the stock waterbox. We have tested numerous aftermarket waterbox replacements and found none that significantly increased overall power.

All Group K Hammer kits are intended to utilize either the “Factory Pipe” brand (on the 701cc engines) or Coffman aftermarket exhaust pipe (on the 633 & 633 Big Bore engines). We do not currently recommend any other pipes for our Yamaha engines.

For all the dual carb Yamaha 701 engine formats, we have had the best overall results with the Factory Pipe Products exhaust systems. Owners looking for an exceptional increase in acceleration should also get the optional ECWI (electronically controlled water injection) system from Factory Pipe. It’s important to understand that while this system greatly improves power from 3500 – 6800 rpm, it makes no change in peak rpm or peak water speed abilities.

Intake Systems

Single Carb 633’s and 701’s – All of the Group K engine kits for the single carb 633cc and 701cc platforms are developed to retain the single carb intake system. Our testing showed us that these engine kits (with a well prepared single carb intake) can easily generate as much speed as most riders would ever need, along with more acceleration power than most pumps can deliver.

The 633/701Hammer 96 and Sleeper kits can both use your existing Mikuni “Super BN” 44mm carb, with internal circuit upgrades. These upgrades feed fuel at the increased rates demanded by the ported cylinder.

The only other single carb arrangement that we found to be a substantial improvement over the upgraded 44 Mikuni was a special arrangement that utilized the 46 Mikuni. The 46 Mikuni and a 3″ tall K&N flame arrestor with a

Jetinetics “turbo cone kit” allowed the 46 carb to accelerate with incredible authority at all water speeds. We consider the turbo cone to be a mandatory part of the 46 Mikuni upgrade for the Hammer kits.

All our Sleeper and Hammer kits utilize a modified intake manifold plate. This modification offers noticeably improved throttle response and acceleration at a very reasonable price (note: this mod makes no change in peak water speed). 633 Big Bore Sleeper kit customers, whose primary interest is quick throttle response and acceleration, we recommend this modification as an added option.

Aftermarket dual carb kits are commonly used on modified race boats because they offer slightly better mid range acceleration than a single carb arrangement. With plenty of time invested in tuning, many aftermarket dual carb sets can be made to operate better than a single in ever, however our testing showed many aftermarket “duals” to be questionably faster and unquestionably harder to maintain than the single carb. Based on these tests, we consider dual carb kits to be a questionably wise purchase for most recreational riders.

Of all the aftermarket dual carb kits we tested on the 701cc Hammer kits, the clear cut best is the dual 42 Kiehin kit sold by Aqua Sports in Torrance, Calif. Kiehins are not commonly used on Yamahas because all the popular makers of Yamaha dual carb kits are Mikuni distributors…not Kiehin distributors. This dual 42 arrangement offers much better acceleration and peak speed than any dual kit we have tested. Since special jetting and mounting hardware is needed, we recommend that customers with limited tuning experience buy the entire kit.

Dual Carb 701cc Platforms – The stock dual 38 Mikuni carbs (on Raiders and post 96 Super Jets and Blasters) are badly obstructed at the air intake mouths by the choke butterflies and other casting bosses. During all our tests, the stock 38 Mikuni carbs (as modified by Group K) consistently offered the best overall acceleration and peak speed of any arrangement tested. Dual 44 Mikuni conversions are heavily promoted and being made widely available. In our testing on the 701cc Raider style engine, dual 44’s worked well in every way. However in back to back tests done on a modified Raider, the Group K modified 38’s offered exactly the same authority of acceleration as the 44’s, along with 100 more peak rpm. Along with the slightly better rpm, the modified 38’s also offered significantly less fuel consumption. As we tested with higher revving setups (using aftermarket exhaust pipes), the modified 38’s had no problem delivering all the mixture needed for high revs, while still maintaining the strong acceleration.

Flame Arrestors – The stock flame arrestor assembly on all the single carb engines works well surprisingly well, and offers good protection from heavy water exposure. For customers who don’t ride in “big water” (that exposes the engine compartment to water) aftermarket arrestors can be an asset. For all the dual 38mm carb setups, we had the best performance results with the R&D arrestor.

Reeds – In all the 633cc and 701cc model Yamaha engines, the stock Yamaha reeds and cages have consistently outperformed any aftermarket reeds we 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 worse than stock.

Many 633cc owners have considered upgrading to 701cc crankcases which hold slightly larger reed cages. While these larger cages may be an advantage on race boats, we have built plenty of “very fast” 701cc race engines that employed the earlier 633cc style reed cages. The advantage of 701 reeds is not monumental.

For the dual 38mm carb 701’s, aftermarket reed stuffers offer a noticeable improvement in overall acceleration. We consider them to be a good horsepower value and recommend the R&D stuffers for these engine packages.

Ignitions – All the stock Yamaha ignitions have a built-in rev limiter. The 633cc model rev limiters “bump” at about 6500 rpm while all the 701 models are set at about 7100 rpm. Raising the limiter on 633 Hammer kits is considered mandatory. However, on the 701cc 96 octane Hammer kits the rev limiter modification is optional. Unlike other machines, these rev limiters cannot be disabled by simply disconnecting a wire in the electrical box. Pro-Tec offers an effective and reasonably priced brain box modification that eliminates the rev limiter. The Pro Tec mod also features a manually adjustable upper rpm limit.

Lightening the stock ignition flywheel on any of the 633 or 701 engines offers slightly quicker throttle response at a very reasonable price. The aluminum “charging flywheels” available from various aftermarket shops are a little lighter and a whole lot more expensive than a lightened stocker. We only recommend aluminum flywheels to racers who have an “open” budget.

Cooling System – All of our Yamaha kits can operate reliably with the stock single input cooling system along with our cooling system upgrades. All our 633/701 kits come with two additional water bypass outlets while the Hammer kit cooling upgrade uses three outlets. All fittings and hoses are included. (note: for customers, who have aftermarket exhausts, our cooling upgrade kits require your exhaust manifold along with your top end parts).

Pump Modifications – 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 conditions (as race boats are). If your machine will be operated primarily on smooth water, the blueprinting will offer little improvement for the money spent.

All pre-’93 Yamahas have a stock bilge system that leaks away a significant amount of pump pressure to accomplish it’s job. Group K offers a bilge conversion for these earlier machines that employs a “no pressure loss” design. This conversion is available only as an option to customers who are purchasing the pump blueprinting.

A Skat Trak or Solas variable pitch stainless steel impeller will be a mandatory modification for all Group K kits. We recommend a 9/17° pitch for all 633/633 Big Bore Sleeper and Hammer kits, and a Solas “I” on all 701cc engines. The Waveblaster and Pro VXR engine kits work well with the stock Yamaha stainless steel prop or the Solas “I”. All 701/753 Raider kits should use the Solas “J”.

Handling – For recreational boats that will be operated primarily on smooth water, cutting out the center bars of the stock scoop grate will offer all the additional water access that’s needed. Aftermarket scoop grates are recommended for all machines that will be run at high speed in very rough water conditions. For riders who run their machine 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 also scrub 1 – 2 mph off peak water speed (4 – 5 off Raider hulls) and often cause “eye opening” deceleration when you release the throttle.

Of all the ride plates and scoop grates we tested on all the Yamaha models, the “Jet Dynamics” and “Worx” components consistently out-performed every thing else on the Super Jet and Blaster hulls. These plates and grates are more expensive than the rest, however in our judgment…well worth the price. For Raiders, We recommend only the stock hardware.

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.

Package / Upgrade / Parts

Group K Price

633cc & 701cc “SLEEPER” KIT – All Single Carb Models

Includes: Cylinder Porting, and Decking, Cylinder Head Modification, Cooling System Upgrade, Intake Manifold Modification and Carb Circuit Upgrade. (SEND Head, Cylinder, Carb, and Inlet Manifold)


Twin Carb 701cc “SLEEPER” ENGINE KIT

Includes: Cylinder Porting, and Decking, Cylinder Head Modification, Cooling System Upgrade, Primer Kit, Carb Boring & Circuit Upgrade. (SEND the Cylinder Head, Cylinder, and Carbs)


BIG BORE Upgrade (701cc – 633s) • (753cc for 701s)

Includes: Cylinder Boring, Piston Assemblies, Bore Head Gasket Set.


753cc – 781cc Sleeve & Big Bore Kit (All)

Includes: Cylinder Boring, Porting, Modify Cylinder Head Dome, Piston Assemblies, Gaskets, Bore Cylinder & Install Sleeves (SEND Cylinder, Cylinder Head, and Top Case) 633cc & 701cc “HAMMER” 96 OCT Kit Components.


Engine Labor & Parts

Group K Price

Cylinder Porting & Decking


Cylinder Head Modification


Crankcase Porting & Matching


Carb Boring & Circuit Upgrade (Dual Carb Setups)


Carb Circuit Upgrade (Single Carb Setups)


Intake Manifold Modification (Single 44 Or 46 Carb)


Hammer Cooling System Upgrade and By-Pass


Ignition Flywheel Lightening (-.4 Lb.) (All Modificationels)


Pro Tec Rev Limiter Modification (All Modificationels)


633 Big Bore Piston Kit – (Piston, Rings, Pin, & Clips)

$138.00* each

Engine Teardown, Re-Assembly & Pressure Test


“Factory Pipe” Exhaust Pipe & Exhaust Manifold (Super Jet, Blaster, Raider)


46mm Mikuni Carb Kit (Jetted Carb, Primer, 3″K&N with Adapter)


R&D Reed Stuffers (Dual 38 Carbs)


R&D Flame Arrestor (Dual 38 Carbs)


Driveline Labor & Parts

Group K Price

Pump Blueprinting (SEND Vain Case And Impeller Housing)


Bilge System Conversion (Pre ’93 Models Only) (SEND Exit And Steering Nozzle)


Skat Trak Impeller All 633-685cc 9/17° All 701cc Conversions 10/18°


Solas Impeller (I Pitch For Super Jet & Blaster) (J Pitch For Raider)


Billet Drive Couplers


“Jet Dynamics” Scoop Grate (S/J & Blaster) (Rough Water Use Only)


“Jet Dynamics” Ride Plate (S/J & Blaster)



Group K Price

Cylinder Boring, Hone, & Chamfering (One Oversize)


Case Machining (To Accomodate Modification 701cc Cylinder)


Engine Teardown & Reassembly


*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 !!