4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Yamaha 1100-1200 – Sleeper Kits for Triples

Group K Modifications

The Group K kits entail modification to existing engine parts only. The finished Sleeper equipped boats will have engine compartments that look 100% cosmetically stock. Our prime objective during development was to increase acceleration as much as possible. Increasing peak speed was a welcome side effect of the work, however it was a secondary consideration. The acceleration of all these Sleeper equipped machines is instant and violent.

NOTE: The Sleeper kit increases in acceleration and speed can have a profound effect on the handling characteristics of these machines. All our test riders agreed that these machines should only be operated by riders who are experienced, physically fit, and “paying attention”. The Sleeper kit triples are machines that demand respect at all times, and should be operated only by riders capable of making consistently mature judgments.

ABOUT PEAK WATER SPEEDS – All the Yamaha triples are equipped with standard issue speedometers. For owners who haven’t already figured it out…these speedos are very “optimistic”. While the instruments often indicate speeds in the upper 60’s, that’s not reality. The following is the accurate peak speed data:



Sleeper – Type 1

1130 – Type 1 – 38’s

1130 – Type 2 – 44 Carbs

Raider 1100

58 mph

63-64 mph

67-68 mph

70 mph

Venture 1100

52 mph

57 mph

60 mph

63 mph

’97 GP 1200*

54-58 mph

+ 4-5 mph


+ 6-7 mph

’98 GP 1200

59-61 mph

+ 4-5 mph


+ 6-7 mph

’98 XL 1200

55-56 mph

+ 3-4 mph



*As we gained access to more GP1200s later in ’97, we found an unusually large range of variation in peak speed abilities among these machines. Since the engine rpms of all these machines do not reflect the same variation, we can only suspect that the range of speeds is in some way related to a hull surface variation. As a result, our speed increases for the GP 1200 are harder to accurately predict.

There are numerous modifications that will net slightly higher peak speeds on glass water. These include, but are not limited to, running only 2 gallons of gas (for weight reduction on the nose), removing sponsons, removing ride plates, etc. On a daily basis, we receive calls from enterprising triple owners who have radared their machines at speeds in the low to middle 60’s using mods like this. While we find these calls interesting, we don’t consider them important. It is not difficult to coax a few additional mph out of a machine if you’re willing to greatly compromise practicality, handling, or costs. At Group K we consider speed increases to be important only if they are a result of sheer horsepower increases. We could easily gain (and advertise) the speeds of a machine with 2 gallons of fuel, no ride plate, and race gas…but that’s not the real world. The real world is where you pull up next to a guy on an XP 800 in afternoon chop, and blow him off. In this part of the real world, there is no glass water and no radar guns. This is the world that we build our kits for.

INDIVIDUAL MODIFICATIONS VS. A “KIT” – Many customers, for time or money reasons, may want to choose individual parts of our modification sets. For these customers, Group K will only offer the head modification as an individual mod. This modification, by itself, can make a noticeable improvement in overall power. However that increase is only a fraction of what the complete kit offers. The dome volume specification of this “head only” modification is the same as the “kit” specification. This means that owners who have the head mod alone, will not have to pay for it again if they later get the Sleeper kit.

IGNITION – We bring up the issue of ignitions early on because of the important differences between the ignition systems of the 1100 and 1200 models. Both systems have a limiter at about 7050 rpm. However the newer 1200 ignition has a different ignition curve for each of the three cylinders. Among other things, the variations in the three curves of this system help to compensate for the varying operating temperatures of the respective cylinders. From a modification standpoint, it allows a modified GP 1200 to run noticeably cooler than an identically modified 1100 that has be fitted with 1200 (84mm) pistons. This is not a big deal on “110 octane race gas” engines. But it is a very big deal for engines that are built to be 92 octane “pump gas” safe. As a result of this difference in ignitions, our pump gas kits for the 1200 will make better overall power than our (pump gas) 1100 big bore kits. For the meantime, fitting the 1200 ignition components onto the 1100 is “availability and cost prohibitive”. Hence, we will not be marketing kits intended for 1100s that might mandate the use of the 1200 electrics.

The aftermarket is full of total loss ignition systems. Because of the constant battery maintenance and the questionable reliability many of these systems have had, we do not recommend them for any recreational or amateur racing applications. Group K will not install or supply a total loss for any engine built in house.

AFTERMARKET CYLINDER HEADS – While these parts are very attractive, we consider them to be questionably worthwhile. The claimed benefits of the billet heads are improved cooling and improved gasket sealing abilities. We agree that these would be very desirable qualities on a high compression engine format. However the stock engines push only 110 – 120 psi of compression. The Group K Sleeper kits for both 1100 & 1200 machines pushes no more than 150 psi. Our testing showed that higher compression ratios were not a measurable performance benefit on our kits for these triples. Many of the billet heads we have seen are made to yield compression ratios that we consider too high to be compatible with 92 octane pump gas. Our testing showed the end result of this choice is a huge increase in operating temperature, with a questionable increase in performance.

The modified stock heads have no cooling or sealing problems that we have observed. Good cooling is a function of efficiently exchanging the heat away, not just a function of the water jacket size. The water jacket surfaces of the stock cast heads are faceted and finished in a way that offers more than sufficient “water contact surface area”. The greater the water contact surface, the more efficiently the head exchanges heat to the coolant. To date, we have seen no aftermarket heads that have more water jacket contact surface area than the stockers.

Another claimed benefit of the aftermarket heads is a greater resistance to “warpage”. Warpage, for the most part, is more a mechanic’s “old wives tale” than a technical reality. No head, on an engine receiving a reasonable supply of coolant, can “warp”. If an engine were run at length with little or no coolant, any head will eventually sustain a distorted head gasket-sealing surface. It is likely that a billet aluminum head would distort slightly less than a cast head. This is because the aluminum mass around the domes is typically so much thicker. Unfortunately this thicker mass also represents a longer heat path that reduces cooling efficiency. In situations where gasket surface distortion is an ongoing problem, the thicker mass head would be preferable.

For owners that intend to get the Big Bore kit for their 1100’s, the “O” ring style heads can have an added complication. Cutting the new larger dome diameter into these heads can often break into the old “O” ring groove…ruining the head. The changeable dome type heads have a similar problem, in that the dome can “drop into” the new larger bore. For all our engine formats, we prefer to start out with the standard head.

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 experience, that may be true. The porting mods used in these 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 prepared as matched pairs to assure that you have the proper squish clearance and compression ratio. These specifications play the key role indicating the amount of overall power that the engine can make, as well as the quality of fuel that you may use.

BIG BORE CONVERSIONS FOR THE 1100s – The Yamaha 1100 has enough sleeve material to be bored to 84.5mm. We offer an 84mm big bore option (allowing for 84.25 & 84.5 oversizes) that brings displacement from a stock 1050cc to 1130cc. Other mandatory modifications for the 1130cc big bore on 1100 models are: true and weld crankshaft, stock rev limiter, 32:1 premix, Solas J prop.

TYPE 1 and TYPE 2 TOP END MODIFICATION – During 1995 – 1996, all our 1100 kits (standard bore and big bore) used our Type 1 porting format. This format offers excellent overall acceleration and peak speed abilities. During the development of our endurance racing 1100 big bore engines, we tested a Type 2 porting format that used slightly more aggressive port timings and widths. On the 38mm carb engines, this Type 2 porting offered a slight gain in peak speed hat came along with a slight loss of bottom end acceleration. However when this same porting was run with 44mm carbs, the increase in acceleration and peak speed was huge. The 44 carb/type 2 engine revved about 300 – 400 rpm more, and had tons of bottom end grunt. The only down sides are “heavy” fuel consumption, and temperatures that mandated the need for 100+ octane fuel. (either “AV” gas or a 50/50mix of avgas and 110 race gas).

For 1997, we will be offering both modes of top end modification. The 92-octane pump gas compatible Sleeper kits for both the 1100 and 1200 will have the type 1 porting layout. For owners that are willing to run the more expensive high-octane fuel, we will be offering type 2 porting for both the 1200’s and the 1100 big bore kits.

While the 1130cc GP 1200 Sleeper can easily operate on 92 octane pump gas, the 1050cc models that are big bored to 1130cc can only be pump gas safe while the modified 38 carbs are used. The extra horsepower generated by the 44 carbs (on Type 1 – 1130cc engines with the earlier 1100 ignition) causes a level of excess temperature that mandates the need for 100 octane fuel. In other words, no 1130cc Group K engines, with 1100 ignition systems, can run the 44 carbs and still be pump gas safe. Note: If there was any way to get the type 2 porting to run reliably on the cheaper pump gas we would do it…please, don’t even ask about it.

ABOUT PISTON CLEARANCE – The original piston clearance setup of both machines is generally very good. Over long-term use, we found significant piston skirt collapsing on both models. In either case, bore wear and piston skirt inspection should be done at least seasonally on engines that experience long term high rpm use. On heavy use recreational packages, we recommend set-up clearances of no more than .004″. Engines intended for moderate recreational use can easily live with .004″ – .005″. Piston clearance inspection is available on request, and cylinder boring is available. We will hone all cylinders before return shipment.

OIL INJECTION SYSTEMS – The stock Yamaha systems inject the oil at a fitting towards the top of the carb mouths. Yamaha chose this injection point so that the choke and throttle shafts would both get internal lubrication from the entering oil. The new 1200’s with the redesigned flame arrestor and variable feed oil pump work great under all circumstances. Unfortunately, (on 1100 models) the changed inlet tract resonation of the porting causes much of this oil to be blown into the engine compartment through the flame arrestor before it can be drawn down into the engine. For 1100 customers who wish to retain their oil injection, we must relocate the oil input fittings from the flame arrestor base down to the inlet manifold itself. Since we remove the choke plates and shafts, lubrication at the top of the carbs is not an important issue.

EXHAUST SYSTEMS – As of this writing there are no aftermarket exhaust systems or exhaust system modifications available for any model except the GP 1200. For addition info about the triple pipe system for the GP 1200, please visit our internet website. At this time, we strongly recommend against any modifications to the stock systems other than “extrude honing” the exhaust manifold and head pipe.

We also recommend that a stock waterbox silencer be retained on all Sleeper kits. Contrary to belief, the stock Yamaha waterbox mufflers yield excellent overall power levels in these high output applications.

INTAKE SYSTEMS – All Sleeper kits include carb jetting, choke removal, and a primer. The 38mm carbs of the 1100 motors also get additional throat boring. This conservative boring mod maximizes the “cfm” ability of the 38 carbs while minimizing additional fuel consumption. Testing done with the larger 44 carbs on these kits results in very significant increases in overall power (good) as well as big increases in fuel consumption (not good). The Group K modified 38’s offer the best combination of power vs. fuel consumption. 38mm carb engines can run about 60 – 70 minutes on a tank at full throttle, verses 40 – 45 minutes for the 44mm carb engines.

Our intake manifold modification is an option available only for the 1100 models. This inexpensive modification results in a noticeable improvement of throttle response and acceleration.

Group K will offer cylinder and head set modification to customers who already have a carb kit from another aftermarket manufacturer. However this will be done with the understanding that all carb tuning and adjustment information will be the responsibility of that manufacturer, not Group K.

FLAME ARRESTORS – For our Sleeper kits, an aftermarket flame arrestor is not mandatory…but it does help a little. In our testing, the durable and reasonably priced R&D arrestors performed measurably better than any other arrangements. It bears noting that the overall performance increase of the R&D is much greater on the 1100 than the GP 1200. For 1200’s, we only recommend the (much lighter) R&D arrestor to reduce the risk of the arrestor coming loose on GP 1200’s that will be run in long and punishing endurance races.

REEDS – The stock reeds and cages on these machines have consistently outperformed 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 often “looses its memory” or begins to chip. As this happens, the performance offered by these weakened aftermarket reeds becomes worse than stock. If you do choose to purchase reed cages or petals from any aftermarket manufacturer, get an assurance that their in-house technicians will be able to help you with the carb tuning needed to get your engine running properly.

The GP 1200 inlet design allows for the use of reed stuffers. For this model, we recommend the Boyesen aluminum stuffers. The 1100 inlet manifold design does not allow for the use of any conventional stuffers.

ABOUT MARINA GAS – There is a great convenience to buying “on water” marina fuel for machines that consume as much gas as these boats do. The unusually high price of marina gas is not a reflection of that fuel’s high-octane quality. The high price is more a reflection of the dock master’s awareness of how captive his customers are. In any case, the normally low grade of marina fuels can work fine in the low rpm, low power output motors in most boats. However, a Sleeper kit triple needs much better octane than most marinas offer. Unless you have personal knowledge of the quality of the marina fuel at your ride spot, we would recommend to not use it for extended full throttle or competition type riding.

COOLING SYSTEM – The stock cooling systems on these triples are, for the most part, sufficient for the Sleeper kits. We have added a water bypass to additionally cool the center cylinder. Our test boats, prepared in this way, logged several hundred “full throttle” miles in endurance racing events with no temperature problems.

CASE PORTING – This is a mod that can make a “noticeable” improvement in overall power that comes without any loss of reliability at all. The case porting is a “detail” modification that is usually done on “cost is no object” racing engines. (note : cylinder is required with top crankcase half to do case porting)

PUMP MODIFICATIONS – The single biggest (bolt on) improvement for rough water hook up for the 1100 machines can be had by installing the Pro Tec Scoop grate. This grate does pull down the motor about 100 rpm because it more efficiently feeds the front of the pump. Unfortunately, this grate will hurt “glass water” speeds. For owners who are interested in sheer peak speed ability, We recommend to cut the center bars out of the stock scoop grate. For the GP 1200’s, the R&D pro series ride plate and scoop grate are the overall best choice. The grate, in particular, eliminate the heavy “nose landing” that the GPs do when the throttle is cut quickly from high speed.

The 98 XL 1200’s have a very “nose heavy attitude in the water. This nose heaviness causes the rider to take excessive punishment at high speeds in rough water, not to mention a noticeable loss in peak speed ability. To reduce both these problems, Group K makes a machining modification to the stock ride plate. This modification makes a noticeable improvement in peak water speed, and high speed rider comfort in rough water (the speedo drive remains intact and functional). We strongly recommend this mod for any XL 1200.

Pump blueprinting also offers much better “hook up” in rough water conditions. It can be an important asset to machines that will be run hard in rough water conditions (as many boats are). If your machine will be operated primarily on smooth water, the blueprinting will offer little improvement for the money spent.

A variable pitch stainless steel impeller will be a mandatory modification for all kits. The 1100 based engines & 1200 Sleepers use the SOLAS “J”. Sleeper owners interested in maximizing fuel economy (without losing speed) should use the new Skat Trak 14/20° Super Slimline. This prop is slightly taller in pitch than the Solas “J”.

The XL 1200 is the only model which we recommend retaining the stock impeller. The XL is badly over-propped in stock form, which works out perfectly for the power of the Sleeper modification.

Solas J Impeller Pitch Variations – The Solas “Super Camber” series of impellers was introduced shortly before we started development on our first Yamaha 3 cylinder Sleeper kits (on the 1100 Raider). We quickly found that the Solas “J” pitched impeller was a perfect match for the 1100 Sleeper kit (allowing rpms in the high 6900s). As the 1100 Venture and GP 1200 models became available, we found the “J” to offer the same ideal results on those models (as well as on our big bore Yamaha 1100 kits).

During late ’97 and early ’98, we began getting reports from 1100 and 1200 Sleeper kit owners about rpms that were uncharacteristically low. The GP 1200 and 1100 big bore kits that were supposed to turn into the middle 6900s, were only able to attain 6700 rpm. After getting several of these reports, we finally obtained a Sleeper equipped ’98 GP 1200 that was experiencing the same problem. We spent a week inspecting and tuning this machine in an effort to recover the lost rpms…but the tuning had no effect.

After exhausting all the other variables, we installed a 2 year old Solas J that we knew turned high 6900’s in one of our earlier test boats. With this older J prop mounted, our ’98 GP 1200 Sleeper turned 6980 rpm, and ran 2.5 mph faster. When we compared the “old J” against the “new J”, there were several very obvious design differences. These differences were the cause of the heavier engine loading (and lower rpms) experienced with these “newer J props”. We contacted Solas regarding the nature of the design change, however they claimed no knowledge of any change at all…yet the newer props are clearly different.

Our next tests were centered around re-pitching the new J in an effort to match the performance of the older J. After several on water tests, we were able to make a specification adjustment to all the impeller blades that recovered all the lost rpm. The unexpected surprise was that the “adjusted” new J ran about 1 mph faster that the conventional “old J”. The acceleration in the mid-range appeared to be better as well.

As a result of these tests, we now perform this adjustment to all the Solas J props used for our kits. We would urge any 1100 big bore, or 1200 Sleeper, customers (running a Solas J) to check their peak rpms with a digital tachometer (on glass water). If your peak rpms are under 6840, you very likely have the new design J that would require the adjustment. The cost for the adjustment is $48, and in house turn around is 2 – 3 days. For customers who purchase a new J from us with their engine kit, the impeller adjustment is done at no extra charge. Please note that we cannot weld of repair damaged impellers. This specification adjustment can only be performed on good condition impellers.

ABOUT GP 1200 SPEED AND HANDLING – There has been considerable controversy about the wide variation of peak speeds among stock GP 1200’s. During our testing we experienced large variations in speed that were caused by stock trim tabs that did not properly fit or seal to the hull. Our tests showed that the stock tabs can account for a loss up to 4 – 5 mph for any GP. Based on these tests, we strongly recommend that the stock plastic trim tabs, on every GP 1200, be replaced with solid aluminum tabs. In particular, we consider these aluminum trim tabs to be mandatory for GP 1200’s that are being modified for more horsepower. We have had good results with the aluminum trim tabs (and optional mount kit) from Riva.

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 kit, you’re welcome to contact us directly for assistance.

Engine Kits

Group K Price

Includes Type 1 cylinder porting, decking, cylinder head modification, cooling system upgrade, carb boring, circuit upgrade, primer kit, & oil injection input relocation. (SEND head, cylinder, carb set, inlet manifold, and flame arrestor)


Includes Type 1 or Type 2 cylinder porting, decking, cylinder head modification, cooling system upgrade, carb jetting, primer kit. (SEND head, cylinder, & carb set)


Includes Type 1 or Type 2 cylinder porting/big boring/decking, cylinder head modification, piston assemblies, top gasket set, cooling system upgrade. (SEND head and cylinder – carb mods not included in this kit)


Engine Labor & Parts

Group K Price









1200 44 mm CARB JETTING








R&D STUFFERS (1200 only)


Cylinder boring, hone, & chamfering (one oversize)


Driveline Labor & Parts

Group K Price



PUMP BLUEPRINTING (send Vain Case and Impeller Housing)












SOLAS IMPELLER (Concord 12/18 or Re-Pitched “J”)




*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 !!