4597 Calle Del Media

Ft. Mohave, AZ 86426

+1 (928) 763-7600


Sea Doo 950 – Technical Updates

Group K Modifications

  • Notice Regarding 950 Engine Kits ————-March ’98
  • R&D Inlet manifold & Reed Stuffers —–December ’97
  • Rave Valve Diaphragms ———————- December ’97
  • Bore Diameter Variations ——————- December ’97


Notice Regarding 950 Engine Kits – We have been continuously testing with the GSXL 950 (gray ones) over the winter of ’97 and spring of ’98. During this time, we have experienced numerous instances of random crankshaft seal failure. While none of these failures has yet led to the damage of other engine components…that eventual risk is certainly there. These crank seal failures have taken place in a random pattern that has made them very difficult (if not impossible) to forecast or anticipate. We would acknowledge that the horsepower increase of our Sleeper kit can increase the loads on some internal engine parts (such as crank seals), however we have never experienced failures of this kind on any other pwc engines we have modified. Bombardier technicians seem to infer that this is not an issue for stock production machines, and therefore a problem unique to machines modified for additional power output. The crank seals of the 950 are of such a unique design, there is not yet any replacements from another engine that can be fitted.

Given this situation, effective 3-15-98, Group K is suspending the sales of all recreational Sea Doo 950 engine modification kits. We will continue to sell 950 engine modification sets to individuals preparing race machines, with the understanding that they must closely monitor the condition of their crank seals.

We are already in the process of testing with the 1998 950 engines (in the XP hull). We intend to conduct our normal prototyping and durability testing process with the new updated ’98 engines. Given our experiences with the ’97 GSXL engines, we intend to conduct “longer than normal” durability tests to assure that there is no crank seal issue for the 1998 design engines. It is our hope to have this testing completed, and begin selling recreational kits, no later than early July ’98.

R&D Inlet Manifold with Reed Stuffers – We tested this manifold arrangement on one of our Sleeper GSXL test machines. This particular test boat was running a Skat 14/19° impeller that allowed for a peak 7050 rpm (hot). We initially had some reservations about the R&D manifold arrangement because it (very visibly) reduced the inlet throat size between the carb and reed cage (compared to the stocker).

In our on-water back to back test, the peak rpm was exactly the same. However the acceleration of this Sleeper GSXL ,which already accelerated far better than a stocker, was considerably better yet. The throttle control during cornering was also improved. We did not test this manifold on a stock machine, however we suspect the low end improvement on a stocker would be much more noticeable that it is on our Sleeper GSXL. For the 950 owner interested in an “all gain no lose” acceleration oriented modification, we consider this to be a worthwhile bolt on.

Rave Valve Diaphragms – The 950 Sea Doo engines use the same (part number) rubber diaphragm to activate the exhaust valve as the 785 rotary valve motors. On the 785 motors we experienced occasional situations where these rubber diaphragms appeared to actually grow about 5 – 10% in size. We later learned that this “growing” was a result of the diaphragms being exposed to cleaning solvents. Once the growth had taken place, the coil spring would have difficulty holding the top of the diaphragm tightly onto the top retaining disk. The diaphragm would typically need to be replaced.

On the 950 engines, we have had this same difficulty with diaphragms that we “know” had not been exposed to any cleaning solvents. When we replaced the diaphragms with new parts, we eventually experienced the same problem with those. When one of these diaphragms pops off, the engine loses several hundred rpm. The diaphragm can easily be pushed back on, but the next full throttle run will push it off again. Slightly shortening the stock coil spring retainer could often fix this problem on the 785 engines, but not so for the 950’s. Standard zip ties cannot be used to retain the diaphragms because the latch head of most zip ties will not allow for full movement of the valve under the cap. We eventually chose to retain the diaphragms with a double loop of .032″ stainless steel safety wire. This double loop does not cut into the rubber material, and it does not come off.

Bore Diameter Variations – We had noticed that a large percentage of the 950 cylinders show uncommonly heavy wear in the area of the bore where the piston rings rest at top dead center. After carefully measuring the bore diameters of these cylinders, we found that virtually all of the bores have .001″ – .0015″ taper at the top of the bore. These bores (in this area) measure slightly “under” the stock minimum clearance spec. That is, the bores were smaller than spec at the tops. We did not measure any noticeable out of round-ness at either the top or bottom of these bores, just the diameter taper. There are no positive aspects to having a bore taper of this kind. By honing these cylinders on a precision hone, the taper could easily be removed. In the course of honing away this taper, most of the heavy wear marks we saw earlier were eliminated too. To be sure, having a non- tapered bore is more mechanically desirable than having the tapered ones. However this bore straightening process does have one unusual (and for us bothersome) side effect. It yields a visible drop in the gauge-measured indicated compression readings. This drop, by itself, is not important. This honing process does not reduce the actual compression ratio. However it does eliminate the effect of the ring end gaps being closed down dangerously close as they approach top dead center (we suspect this is the cause to much of the heavy wear that exists at the tops of the bores). The elimination of this excessive ring end-gap tightening is what causes the reduction of gauge measured indicated compression numbers.

Given this, it’s impossible to make indicated compression comparisons without first assuring whether or not any existing bore taper has first been eliminated. Based on the cylinders we have seen, we cannot say that “all” GSXL cylinders have this taper…but a “whole lot” of them do.

Determining whether a bore has taper requires accurate measuring tools used by someone with experience. If you do find taper in a bore, the bore straightening must also be done on a precision hone by a technician that knows how to use it. All 950 cylinders sent to Group K for Sleeper modification will be inspected for taper, and honed to eliminate any taper that exists. We ask that customers be wary of the reductions in their gauge measured indicated readings that will result from this bore straightening.