4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Sea Doo XPL 950 – Engine Mods

Group K Modifications

While the driveline in the ’98 Sea Doo 950 models is an updated version of the 97½ GSXL model, the differences are significant enough to merit a separate document. As of this writing, the XPL is still considered a “recently released” model. Ordinarily, we will test for several months before releasing data or offering modifications. However the overwhelming interest among ’98 950 owners (XPL owners in particular) for modifications has motivated us to release the information we have to date. While all the 950 models have been popular, the data in this document will be specific with our testing of the XPL model. As new information becomes available, we will post that data to out 950 Updates Document on our website: www.groupk.com/

Objectives – The XPL is a top line high performance machine used for many different applications by owners. Given this, we focused on modifications that would yield an increase through out the entire power range (not just acceleration or just peak speed). In addition to this, we developed our modifications in a way that does not compromise the rideability or the ease of operation. Both the Type 1 and Type 2 packages offer significant increases in acceleration as well as peak speed.

During the course of our testing, we raced our Type 2 XPLs in several region 1 endurance events. These XPLs were clearly the fastest open pro machines in the fields we raced in (not to mention that we got better fuel economy than stock XPLs as well). These same “Group K Type 2” modification sets are now being used, on XPLs in selected events, by pro tour racers Chris Fischetti and Karin Paturel.

The Kits – As of 6/98, our modifications kits for the XPL are as follows:

Type I

• Head Modification and cooling system upgrade

• R&D Reed stuffers and arrestor

• Aqua Sports 42 Kiehin dual carb kit

Type 2

• Group K Head Modification and cooling system upgrade

• Type 2 Cylinder porting

• Aqua Sports 42 Kiehin dual carb kit … or • Novi dual 44 Mikuni carb kit

• R&D Reed stuffers and Arrestor

• Micro Touch cdi module

RPM (stock prop) Speed (stock prop) Octane Requirement

Stock XPL 6840 61-62 mph 87

Type 1 kit 7040-7080 64-65 mph 92

Type 2 kit 7220-7280 67-68 mph 100

As our testing for the XPL went on, we encountered two fundamental problem areas. One is the high performance “unfriendliness” of the oem carburetors, and the other is the unpredictable longevity of some lower end hardware parts. We urge any owner, considering modification to their ’98 950 platform, to consider the information below in the sections marked “Carburetion” and “Side Effects” before purchasing modifications

Compression – Increasing compression ratio is always a very popular first modification, and the XPL responds well to “subtle” increases in this area. The use of compression gauges (to establish a measurement in “psi”) has been a very popular way for owners to compare compression. As the “Compression” document on our website points out, measuring compression in this way is, at best, a “ball park” measurement with limited meaning. It the case of the 950 Sea Doo, indicated compression measurements are even more worthless than they have been on earlier pwcs. The primary reason for this is cranking speed.

Accurate comparative indicated compression measurements must be taken at cranking rpm speeds that are as close together as possible. For pwcs with smaller displacement individual cylinders (which in this case is “all” pwcs), it’s not too difficult for the starter to maintain a relatively narrow range of starter cranking speeds over a very wide range of indicated compression numbers. However the starter in the 950s has a lot more work to do than any other pwc starter. If a very high compression head is installed, a compression gauge will not be capable of showing an accurate measurement of the increase because the extra load of compression significantly slows the starter cranking speed. On the 950s, we have seen cylinder heads of several cc’s difference show indicated numbers only a few psi apart, while the accurate difference would have been 20-30 psi. All this said, “relative” indicated compression measurements might be a useful diagnostic tool on an otherwise unchanged 950 motor. However discussing various 950 compression ratios in terms of indicated psi is (in our experience) a total waste of time.

All this said, the compression ratio increase of our 950 kits is “very” conservative. Our testing showed that very strong overall acceleration can easily be accomplished without big increases in compression ratios.

Included in our head modification is a cooling system upgrade. The 950 is the only pwc engine we have seen that can “land lock” air pockets in the water jacketing around the combustion chambers. If this air pocketing happens during high speed operation (on a modified motor), it can cause localized over heating that can easily lead to detonation. Our cooling upgrade completely eliminates this air pocketing.

While restricting water to various parts of the exhaust system has been a popular modification on many other machines, we found very questionable benefits with this type of mod on the XPL. For us, the added risk of burnt exhaust couplers simply didn’t merit the small potential increases in peak rpm.

Please note that our technicians are not permitted to discuss the specification information about our cylinder head and cooling upgrade modifications.

Carburetion – The “I” bodied Mikuni carbs used on the ’98 950s are a re-designed version of the ’97 GSXL carbs, however both versions share the same primary design features. The carbs used on all 950s are the first generation of a carburetor that is designed as much for emissions friendliness as it is for performance. Among the most notable design features that makes this difference, is the significantly increased distance between the butterfly and the “booster venturi” style fuel atomizer (aka “bombsight” atomizer). We cannot claim to know all the effects of this design departure, however we know a lot about one particular effect called “fuel delivery signal”.

One of the most desirable features of the carburetors used on pwcs is that they are “load sensing”. That is, pwc carbs read the varying vacuum “signal” from the lower end, combined with the signal created by the air flow passing the booster venturi, to deliver the correct amount of fuel for a particular level of load (and butterfly position). The increased distance between the butterfly and the booster venturi on the 950 carbs significantly changes the way the carburetor “sees” that signal. On completely stock machines, this does not appear to be a significant problem. However as horsepower output (and rpms) are increased on the 950 motors, the stock carburetor can have a tendency to “lose signal” altogether at certain rpms. The result is a steady drop in fuel delivery when the engine is sustained at one particular rpm. This particular rpm can vary based on the engine’s powerband and rpm abilities. In the case of the ’98 950s we experienced this phenomenon at 6700 rpm.

During our on water testing, we monitored fuel flow with a digital fuel flow meter. On our Type 2 kit (using rejetted stock carbs) the boat performed flawlessly in every “high performance” way. Our test boat could easily yield excellent acceleration and run at 7000+ rpm all day. Our fuel flow meter would show the steady and stable fuel delivery rates at every rpm in the range … except 6700 rpm. When the throttle was held to 6700 rpm (on a digital tach), our test rider could watch the fuel flow steadily declining … until the engine suddenly went into detonation and then piston seizure. Since this lean condition was being caused by the carburetors inability to “see signal” at that rpm, no amount of jetting could solve the problem. While 6700 is not a common cruising rpm, it would only take a few seconds of operation at that “magic” throttle setting to send the engine into detonation. We conducted similar fuel flow tests on our Type 1 kit, and found “some” machines that lost signal at 6700 … and some that didn’t (we still don’t know why).

We experienced this same kind of problem during the development of our 97 GSXL kits …at 6500 instead of 6700 (we have no idea why the difference). We were able to restore the signal of the 97 carbs, at the problem rpm, by making a carb throat modification. However, for reasons we cannot explain, the same throat modification (and others) have not resolved the problem for the ’98 carbs. As a result of all these tests, we are recommending a carburetor replacement for any 950 owner looking to significantly increase the performance of a ’98 950. Our recommendation as the two most effective replacements are the Novi “XR 44” carb kit (these are modified spigot mount 44mm super BN Mikunis), and the Aqua Sports 42mm Kiehin kit. Both these carb kits offer better acceleration and rpm, not to mention eliminating the signal issues connected with the stock carbs. Unfortunately, they both also eliminate the oil injection feature of the 950 (we recommend a 32:1 premix).

About Carb Throat Size — While the stock 950 carbs are denoted as “46mm”, the actual minor diameter at the main venturi is slightly under 42 mm. With respect to pwc carburetors, there has often been a mindset of “bigger must be better”. In the real world (and the 950 world) this is simply not the case. When Novi Performance conducted dyno tests of their 44mm XR carbs vs. the XR 46mm (on a stock 950 engine), they obtained the same horsepower at peak rpm … with an 8 horsepower increase at 5000 rpm. Our own “on water” carb tests consistently confirmed that our Group K modified 950s yielded better overall power with slightly smaller throat carbs. In addition to this, the full throttle fuel consumption was actually less than stock.

Of the two carb kits we recommend the Novi is the more expensive … but with good reason. These carbs mount to the engine via durable rubber spigots (as motocrosser carbs do), and are connected to one another via a fuel rail that doubles as a fuel/air separator. This separator feature allows for “bog free” high speed turning, right down to the last ounce of fuel in the tank. The carbs themselves are extensively modified Mikuni super BN 44s that are fed by a single remote Mikuni fuel pump. As of this writing, the Novis cannot be used with the stock 98 XPL throttle cable, hence the (somewhat expensive) 97 GSXL cable must be used (we expect that Novi will change this soon). The Novi carbs will accept the popular one piece R&D flame arrestor (which is our top choice).

The Aqua Sports 42 Kiehin kit, while more affordable, is no less effective. The Kiehin carbs each have their own pump, so an additional pulse must be fitted into the case to drive the second pump. The Kiehins will also accommodate the stock throttle cable and the R&D one piece arrestor.

In our Type 2 tests, the Kiehins performed every bit as good as the Novis and got slightly better fuel range. However we suspect that as slightly steeper props become available, the Novis will likely pull them with more authority than the Kiehins. The Novis are the better choice for owners planning to build full out modifieds.

Cylinder Porting – The Group K cylinder porting for the XPL 950s is by no means “radical”, but it does yield an impressive increase in overall power. As noted above, the porting allows for about 200 additional rpm over the Type 1 kit. The increase in acceleration is equally impressive. Because of the way we profile and chamfer the 950 ports, piston and ring wear are every bit as good, if not slightly better than stock.

Our 950 cylinder porting is available in two different finish modes. The “competition finish” includes all the port shaping, specification work, and passageway final surface finishing. This final finishing work accounts for about 30% of the total time (and cost) yet contributes to only about 10% of the total performance gain. The recreation finish (used in the Sleeper kit) offers all the fundamental dimension and port shaping work, without the more costly fine finish work. The cost difference…$440 vs $320. Which-ever finish you choose, we require the cylinders with the complete Rave valve assemblies mounted in them. All ported cylinders are also honed.

Ignition – Through all our tests we found the stock 950 ignition to perform problem free. Based on our water tests, the limiter in the stock ignitions appears to be in the 7120-7150 rpm range. The Type 1 kit can easily operate with this stock rpm limit. However the Type 2 has the ability to significantly over run the limiter. For the Type 2 kit, we recommend the Micro Touch cdi Module which elevates the limiter to about 7500.

It bears noting that the limiter has a purpose that goes beyond simply allowing for more rpm. During high speed riding, in rough water conditions, the pump occasionally comes disconnected from the water allowing the engine to rev up to the limiter. When then engine “bumps” the limiter, the ignition delivers a “performance unfriendly” firing that reduces engine rpm. If your pump reconnects with the water while this “unfriendly” firing is going on, it will give the rider the feeling of “landing with the brakes on”. When the cdi module is installed, the engine is allowed to momentarily rev to a higher rpm with “still friendly” firing that will offer the feeling of “landing with the throttle on”. This is a very important feature for closed course and endurance racers.

Impeller/Pump – As of this writing there are no commercially available ’98 950 impellers. Both of our kits perform well with the stock prop, however the more powerful Type 2 can easily stand to have the stock prop made a “little bit” steeper. There are several pwc impeller shops that offer this service. Our testing showed that the type 2 liked to run best in the middle 7100 range.

Side Effects – We can certainly appreciate that Sea Doo’s primary interest (like that of all pwc makers) is to focus on making their machines operate reliably in totally stock form. Given that, they construct their machines only to endure the stresses and loads of stock output and stock speeds.

At Group K we fully concede that no increase in performance comes “with no luggage”. We conduct lengthy and abusive testing to uncover as much of this “luggage” as possible. This allows us to either fix a problem in advance, or offer owners a clear idea of changed maintenance needs. The performance and rpm increases of these 950 Group K kits are comparable to the increases we have made on numerous recreational pwcs for many years. However the “side effect” issues we experienced during the testing of these high performance 950s are more profound than anything we have experienced in the past.

The bulk of our endurance testing difficulties have been centered around the crankshaft seals and big end rod bearing hardware. While these parts are themselves not very expensive, the residual damage of their failure can often inflict costly problems.

During our testing of the 97 GSXL, we experienced an uncommon rate of random and unexplainable failures of the crankshaft seals. Most common were the front crank seal (under the ignition cover), and the center crank seals (which breech into the center balancer drive area). Since both of these breech areas are reasonably air tight areas, these air leaks do not carry the immediate consequences that a normal 2 cycle lower end airleak might have. However, as we often tell all our customers, air leaks never heal … they only get worse. Until we find higher quality seals to the place the stock seals, we would recommend that owners of high output 950s pressure check their engine every 6-8 racing hours. Pressure test kits are available from Watercraft connection in Oregon (503-232-2062).

We are told, by our Sea Doo parts experts, that the crankshaft seals in the ’98 models are unchanged from the ’97 models. We cannot say that this means that the seals in stock ’98 machines will experience the same “difficulties” we saw in our ’97 GSXL test units. But, to us, it does mean that the difficulties we experienced with the ‘97s has a good chance of showing up on modified ’98 units.

Beyond the issue of crank seal longevity, we have also experienced fracturing and disintegration of big end rod bearing cages and big end rod thrust washers. Despite our use of 32:1 fuel/oil premixes, these bearing cage and thrush washer failures have also taken place in a random and unexplainable pattern (some do…some don’t). It is important to point out that these failures (in our opinion) are not related to a poor design, but rather related to materials quality that may not be up to the rigors of significantly increased rpm or power output.

With this situation, we realize that the long term reliability of Group K 950 kits (as well as our future kit sales) will hinge on our ability to locate and re-fit higher quality components (seals, cages and thrust washers) in these areas of “difficulty”. We are currently working on the replacement, and durability testing, of higher quality material replacements for these problem areas. We consider this task to be very important since none of our Type 2 test units (to date) have operated at competition speeds for over 10 hours without experiencing one of the afore mentioned “issues”. As we find lasting solutions, we will post the updated data to our website.

Type 1 Kit

Group K Price

Cylinder Head Modification and Cooling Upgrade


R&D Reed Stuffers


R&D Plenum Flame Arrestor


Aqua Sports 42 Kiehin Carb Kit
(carbs, linkage, manifold, R&D Pro-Lock arrestor adapters, and primer)


Type 2 Kit

Group K Price

Cylinder Head Modification and Cooling Upgrade


Type 2 Cylinder Porting and Decking (recreational finish)


Type 2 Cylinder Porting and Decking (competition finish)


R&D Reed Stuffers


R&D Plenum Flame Arrestor


Micro Touch CDI Module


Aqua Sports 42 Kiehin Carb Kit
(carbs, spigots, linkage, fuel pump)


Novi 44XR Mikuni Carb Kit
(carbs, linkage, manifold, R&D Pro-Lock arrestor adapters, and primer)


97.5 GSXL Throttle Cable (for Novis)


*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 !!