Sea Doo 785 – Technical Updates

Rave Valve Diaphrams – Recently a customer had brought us his ’96 XP 785 Sleeper complaining of a big loss of peak rpm ability. All the engine internals seemed to be in good condition, and the engine did not respond to a wide range carburetor adjustment.

After checking all the normal stuff, we decided to see if the rave valves were getting full movement. One quick way to confirm this is to run the machine (on the water) after completely removing the black plastic rave adjuster caps and springs (on the cylinders). While the bottom end performance is not very good, the valves are certain to attain the pull up position. With these caps off, our customer’s boat immediately picked up the 200 rpm it had been short of. Reinstalling the caps resulted in the 200 rpm loss returning. After plenty of inspection and parts changing, we isolated the problem to the rubber diaphragms on the rave valves. These diaphragms were well secured on the top and bottom, and visually “looked” good. However when we replaced them with new diaphragms, the engine ran (and revved) normally with the black adjuster caps and springs in place.

Apparently the rubber rave diaphragms can become weakened in a way that allows them to look okay…but not work okay. This particular machine had a lot of hours on it, so we figured it is just a matter of time before it happens on most rave engines. If you have a 785 rave boat with lots of hours on it…it would pay to change these diaphragms before you set out for the season.

Rossier Pipe Tests PART 2 – While we were pleased with the 92 octane tests of the Rossier pipe on the ’96 XP hull (with the Sleeper kit), we realized that the 785 models needing the most help (performance wise) are the heavier and longer 785 Sea Doos. In particular, the badly under powered 1997 spring seat 785 XP. We installed the Rossier pipe on a ’97 XP 785 (with the Sleeper kit and stock prop) to see if we got the same performance numbers offered by the lighter ’96 XP. It bears noting that the rev limit eliminator module for the ’97 ignition is a little over twice the price as the similar module for the ’96 electrics. ($228 vs $99).

On the water, the peak rpms of the ’97 XP Sleeper were the same as the ’96…7300 rpm. Unfortunately the loss of peak rpm, that came with extended full rpm operation, was also present. One big difference, however, was that this ’97 XP setup had noticeably less authority of acceleration off the corners (compared to the ’96). This weaker acceleration was on the borderline of being acceptable for recreational riding, and was definitely not acceptable for competition of any kind. We considered this a problem that had to be resolved.

In an unrelated conversation with Gary Robison (of Novi), we mentioned this acceleration problem. Gary noted that he had solved such problems with slight increases in carb throat diameter. We explained that we had never see any such increases in the same tests we had done with our Sleeper kits (using the stock pipe of course). Gary agreed with us with respect to the stock pipe tests. However he stated that the installation of higher rpm biased exhaust pipes can change the inlet tract resonation in such a way that larger throats can offer a genuine benefit. He also denoted that the stock inlet manifold opening is usually 41.5 – 41.8mm in diameter. To this end, Novi does a modification (for about $240) where they bore the stock carbs to 42mm, and fit them with larger butterflies. We immediately got a pair of these 42’s, and installed them on our ’97 XP Sleeper/Rossier pipe test machine. The increase in acceleration was phenomenal. The larger heavier spring seat 785 bolted off the corners every bit as good as the lighter ’96 boat had. There was no increase in the 7300 rpm peak, however it seemed that the boat didn’t loose near as much peak rpm ability during the extended full throttle runs. All in all, this package made the ’97 XP a very powerful machine.

After running these tests, we began to wonder if the 42 Novis might offer a similar kind of gain (with the RE pipe) when installed on our lighter ’96 test boat. Within a few hours, we had the RE pipe and the 42s mounted on our ’96XP ready to hit the water. Again, the peak rpm remained around 7300…but this machine was fast…very, very fast. The acceleration was easily acceptable for competitive level closed course racing…on 92 octane. The acceleration of this setup was so violent that the boat was a little hard to control. We installed a set of (smaller) R&D 85/88mm nozzles with hopes that this would tame the boat down a little. We expected a reduction of peak rpm…but there was none. Instead the XP ran right up to 7300 again…but picked up 1.5 mph doing so. The acceleration seemed to be a little more controllable…but only a little.

The end result of this test certainly delivered more than we expected. This ’96 XP setup (Sleeper/RE pipe/Novi 42s/ R&D nozzles/stock prop) is the quickest and fastest pump gas pwc that we have ever built. Depending on weather conditions, it radars between 64.3 and 65.2 mph. Every one of our test riders considers it the ultimate recreational pocket rocket…and the fastest pump gas boat that they have ridden.

Rossier Pipe Tests – After 2 years of selling Sea Doo 785 Sleeper kits, we have been overwhelmed by kit owners who are looking for “a little more” speed while staying 92 octane pump gas safe. The deteriorating “high performance friendliness” of pump gasolines has made this request ever harder to accommodate. The area most kit owners are interested in approaching for this increase is that of aftermarket exhausts. We have tested repeatedly with the FPP spec 1 and 2 pipes (that we use in our race gas kits) in an effort to come up with a “faster than a Sleeper kit” pump gas setup. To date we have been unsuccessful in this area.

After several conversations, Charlie Rossier (of Rossier Engineering), convinced us to try one of his pipes along with our Sleeper kit 785. Charlie swore that his pipe would allow for the extra speed we were looking for, without inflicting the rear piston scoring that had accompanied so many other such tests with other pipes. We had tested the Rossier pipe on our race gas engine packages, but ended up choosing the FPP spec 2 pipe for it’s higher rpm abilities on those 110 octane setups (about 150 – 200 more).

For these pump gas setups, Rossier Engineering normally subscribes to the Skat Trak “swirl” props (for their excellent rough water hook up). Unfortunately these, great accelerating, props normally make for a loss of 2 – 3 mph of peak speed on smooth water. Our customers were clamoring for more smooth water speed, so using the Skat swirl was not an option. Just the same, we installed the Rossier pipe on our in-house ’96 XP Sleeper boat. This machine runs high 62’s to low 63’s (mph) with a stock prop and 85/88 nozzle @ 7090 rpm. To be a success, the Rossier pipe would have to generate speeds above this set up.

We started our testing with a stock prop and stock nozzle set (later testing would show this to be the best setup for peak speeds running the RE pipe on this boat). After all the jetting work was completed (setting the high speed richer until the rpms dropped), we considered our XP Sleeper safe to run on long, wide open, glass water test runs. The cold rpm numbers (stock prop & nozzles) were always around 7300…very fast (about 64.5 mph). However the rpms began to drop noticeably after 2-3 minutes of w/o glass water operation. After 6 – 8 full throttle miles (and minutes) the rpms would drop to a sustainable low of 7100 – 7120. This “hot number” rpm yielded a peak speed well under the smaller nozzled “stock pipe” Sleeper. Despite the huge loss in hot tach numbers, we were still very encouraged by the potential of the pipe’s cold tach numbers. We were also very encouraged because this was the first setup we had run, at these rpms on pump gas, without scoring a rear piston. However for the Rossier pipe-ed setup to be truly better (speed-wise), we needed to sustain the hot numbers at least in the low 7200’s.

In an effort to reduce the loss of hot rpm numbers, we tested with lower compression, milder ignition timing, various props, nozzles, etc. However, anything that improved the hot tach numbers, also reduced overall performance. Any pump mods that pulled the rpms down, hurt the low end acceleration to an “unacceptable” level.

After exhausting the mechanical possibilities, we attempted to resolve the temperature problem with some octane improvement options. Using ordinary over the counter octane boosters had almost no effect at all. A 50/50 mix of 100 octane avgas/92 octane pump gas allowed the engine to stabilize at 7160 – 7180. A 100% batch of 108 octane race fuel allowed for sustained 7300 rpm operation (without any rpm loss at all). In short, nothing affordable worked very well.

At this point it bears noting that all our ( 92 octane) tests, of extended full throttle on glass water, were intended to portray the heaviest possible use. Very few customers actually operate their machines in this way. With this in mind, we conducted tests to see how much rpm could be recovered if the machine was allowed to cool down via low speed operation (after the 10 minute w/o pass). We again ran the motor from the cold 7300 rpm down to the fully heat-soaked stabilized 7100 rpm. Without shutting the motor down, we then ran the boat at low to medium throttle for about 5 minutes in an effort to reduce the engine temperature. After this cooling period, we once again pushed the engine up to full rpm. This means of engine cooling allowed the rpms to run in the 7230 – 7250 range for a minute or two before the rpms once again descended into the low 7100s. No matter how many times we repeated this process, the cool off period would always allow for the short spurt of returned middle 7200’s.

Summary: Unlike other exhaust systems we have tested to date with our 785 Sleeper kits, the Rossier pipe appears to offer safe (non piston scoring) operation into the 7200 – 7300 rpm range on 92 octane fuel. As long as the rpms are above 7200, this pipe allows the Sleeper to run slightly higher radar speeds than the best stock pipe Sleeper setup. This 7200 rpm range can remain within reach (with 92 octane) so long as the engine is not run “constantly” at full load. Owners who seek to run their engines at 7300 (indefinitely) can do so by simply using 105+ octane race fuel.

The loss of rpms that comes with engine heating is not a function of any design problem with the pipe. It is a function of the sheer heat being generated by the higher horsepower output. The only way to eliminate this problem is by cool down running or race gas use.

NOTE: All these tests were conducted on a ’96 XP boat. We intend to repeat these tests on the larger, heavier, GSX and ’97 XP hulls as time permits. We will post those results as an addendum to this entry.

Coffman Exhaust Pipe – We had an opportunity to test a production Coffman pipe on one of our GSX endurance racer boats that was using our modified stock exhaust pipe (our modified stock pipe allows for 200 rpm over the stock unit). With this modified stock pipe, our test unit turned 7280 rpm with an Xo prop and a stock 87mm exit nozzle. After installing the Coffman pipe, the same boat accelerated much harder in the bottom and mid range, not to mention having the power to pull a 2mm smaller exit nozzle to the same 7280 rpm (larger nozzles turned more rpm, but did not make more speed). This would equate to about 1.5 mph gain over our modified stock pipe(that’s pretty good). The Coffman pipe was easy to install and easy to tune in. The headpipe fitting uses a Mikuni main jet for the adjustment of water being injected into the pipe. We got our best results with a #115 jet. Given that the water injecting point is so small in diameter, a water filter would be mandatory to avoid accidental blockage (no filter came with the pipe). Talking about jetting, this pipe also caused our GSX engine to want bigger pilot jets (85’s from 77.5’s) than it has ever needed before (even with the 7500 rpm FPP Spec 1)

The stamped aluminum body was extremely lightweight, and easy to get in and out. Our endurance racers had some reservations about the long term abuse that this pipe body might be able to endure…but only time will tell.

What everyone wants to know is, “how does it compare to the FPP pipe”. Well this same machine, fitted with the FPP spec 1 pipe, turned 7500 rpm with the same prop and nozzle combo. From this respect, the FPP pipe yielded higher speeds…but it has to turn 7500 to do it. Installing a taller prop to pull the FPP setup down to 7300 hurt waterspeeds and rough water hookup (see Impeller Selection and Water Velocity above). If your a closed course rider who wants the added reliability of the more conservative rpm peak, the Coffman can be a good choice. However, we don’t see the Coffman design being tunable to get the strong 7500 rpm that the FPP makes.

It should be noted that neither of these rpm ranges can be considered anything but “race gas only” territory. If you want to run 92 octane pump gas…forget about all of them (our modified stocker included).

Pump Exit Nozzle Diameters – During the prop and nozzle testing mentioned above, we noticed that our best rpm numbers and radar speeds were made with exit and steering nozzles that had a 3 mm diameter difference. When we finished the testing, we called R&D to let them know. They had apparently learned the same thing, and had already started production of 85mm/88mm and 82mm/85mm nozzle sets. For owners of earlier nozzle sets, we recommend that you measure the exact diameter of your exit nozzle, then bore your steering nozzle exactly 3mm larger…it works.

R&D Trim Tabs – The R&D trim tabs (aka “side wings”) are a great benefit to XP owners who do alot of riding in exceptionally rough water. These tabs help keep the nose of the boat down, not to mention a big reduction in proposing. The down side of these tabs is a 2 – 3 mph loss of peak water speed on smooth water. Since many XP owners are paying lots of money to “gain” 3 mph from their engines, a loss of this magnitude is not acceptable. We recently prepared a Super Stock 2 kit on an XP for Karin Pautrel to run in an endurance race. We wanted to use the R&D tabs, but we didn’t want to lose the speed.

We laid our stock trim tabs against the bottom of the R&D replacement, and scribed the profile of the stock tab onto it. We then machined the bottom of the overhanging area of the R&D tab at an upward 3 degree angle. The results were impressive. The hull maintained excellent rough water stability, along with less than a 1 mph loss on smooth water. We are made to understand that R&D now inventories these trim tabs with this same 3 degree cut. We strongly recommend them to anyone considering a trim tab purchase. As for the older tabs, we won’t have the shop time to start machining them for customers until July (yes we are that busy).

Precision Top End Assembly of the Laydown Rave Motors – Squish clearance, the distance between the piston crown and cylinder head at top dead center, is an important specification that is minded by the Rotax factory as well as all high performance engine builders. The Sea Doo manual offers a fairly wide tolerance for squish clearances, however most engine builders prefer closer clearances to help stave off detonation. Most builders measure the squish clearance by sticking a piece of solder through the spark plug hole toward the outside edge of the bore…then momentarily touching the start button so the piston makes several strikes at the solder. This allows the solder to be crushed to the exact thickness that shows the “squish clearance”. At Group K we make a point to take these “squish clearance ” measurements over both ends of the wrist pin. This minimizes the piston’s ability to “rock” and show an inaccurately large clearance. We also take these measurements over both ends of the wrist pin, on both cylinders, to assure uniform clearances. In a perfect world, all four squish measurement will be within .002″ – ,003″. Unfortunately, we have been observing everything but uniform clearances. This is a matter we do not take lightly, since accurate squish clearance is the only way to assure correct deck height setup (a function of the base gasket thickness).

Immediately, we though that the variations were caused by incorrectly cut squish bands on the cylinder head…not so. The bands had a uniform depth and perfect center to center location. We then suspected variations in the locations of the cylinder mounting holes in the crankcases, but they also measured perfectly. In the end, we found two items that caused the variations. The first was that the distance between the two cylinder bores can vary greatly, depending on the assembly procedure. That is, there can be almost 1 mm variation in the center to center distance of the bore diameters based solely on the random location during assembly. Secondly, the cylinder head itself has a great deal of movement leeway (both left to right, and forward to back). All these variations of fit take place because the cylinders and cylinder head are not located to one another with “dowel locating pins” (as most Kawasaki engines are). The absence of these locating pins allows for alot of movement leeway, of both the cylinders and the cylinder head. That results in broad variations in squish clearance measurements. Installing dowel pins is a questionably wise solution because of the great risk that the exhaust manifold faces would not be perfectly parallel (that would cause bore distortion when the exhaust manifold is torqued on).

For now, our best solution is to recommend a “Precision Assembly Procedure” that will make for minimum variations in squish measurements.

Our procedure is based on the following information. The cylinder base bolt patterns in the cases are located 132mm (5.196″) apart. The standard bore diameters of the cylinders are 82.0 – 82.08 mm (3.228″ – 3.231″). This means that the inner distance between the two bores of the torqued on cylinders should be between 49.91 – 49.98 mm (1.965″ – 1.968″) These last measurements can be taken quickly and easily with a set of dial calipers after the cylinders have been torqued on. In most cases, we have found that you can hit this spec range if you install the cylinders as close together as possible. It bears noting that we have seen engines where the water jackets of the cylinders touched before we could reach this specification range. We literally needed to belt sand some material off the aluminum cylinder casting to get the correct spec. Cylinders like this are certainly the exception (not the rule), but they do exist.

Once the cylinders are installed with the acceptable center to center distance, it becomes apparent that the cylinder head itself can move about 2mm (.080″ in any direction over the tops of the bores. This location will also have a profound effect on squish clearance measurements. We got out best results by matching the edge of the head casting, all the way around, as closely as possible to the edges of the cylinder castings. While this doesn’t sound very precise, it works surprisingly well. After torqueing on the head, you can take your four squish clearance measurements to determine if the head needs to be moved slightly one way or the other…it’s not really as tough as it sounds.

Does everyone one who assembles a Rave top end “have” to go through this whole procedure…not necessarily. However we strongly recommend this procedure to anyone preparing a high output Laydown Rave motor. If you find a wide variation in the squish clearances of your freshly assembled engine…it could certainly avert some problems before they get started.

About the Factory Pipe Products Pipes – All of our early development work for our 785 laydown rave racing engine packages was performed with the use of an early version of the FPP Spec I pipe (the one photographed on page 45 of the Feb ’97 Splash). This pipe, used without the ECWI, made great overall power and acceptable “full speed left” tach numbers. However it did appear to need a relatively low impeller pitch to accomplish all this on our engine packages. These low impeller pitches resulted in rpms (and octane numbers) that was acceptable for our Super Stock II formats, but a little on the high side for our Super Stock I kit. The S/S I kit is intended to be a 100 octane format that could be quick enough to be a competitive closed course racer, yet still withstand the long term abuses of off shore racing. For this, we needed an rpm peak of no more than 7250. We found that by modifying the components of the stock pipe body, these rpms, along with impressive midrange acceleration, could easily be attained. In fact this arrangement had so much overall torque that we were able to use the higher pitched Solas X prop with an 85mm nozzle. This new pipe/prop setup revved 150 rpm less than the Spec I/Solas x setup, yet allowed for the same peak waterspeeds. This modified stock pipe cannot turn the 7500 rpm’s needed for the S/S II, but we think it is an ideal choice for our S/S I kit. Since we do no testing of parts on otherwise stock machines, we have no information for the performance (or prop choices) of the FPP pipes on a stocker. However, it bears noting that Factory Pipe Products developed their pipes on a stock engine…not a Group K kit. It is no surprise that our test results differ somewhat. As for our current S/S II kits, we recommend exclusively the FPP Spec II pipe. On our test boats, the Spec II out performed the Spec I in every way. We suspect that the Spec II pipe worked much better in this application because it was developed on modified engines as well as stockers.

Loose Rave Valve Tops – The Rave valve guillotines are attached to the actuating diaphragms via a plastic threaded cap. The top spring rests between this threaded cap and the top cover. The Sea Doo manual does not give a torque spec for the tightness of this threaded cap, however we presume that the torque must be relatively light to avoid peeling the threads out of the plastic cap. If this threaded cap unscrewed, the guillotine would stop itself before running into the piston. However, during the process of unscrewing the guillotine would protrude noticeably into the exhaust port (at the full up position). That is, it would not be moved into the full up position flush with the exhaust port roof. Among other things, this could cause a noticeable loss of peak rpm ability. Knowing this, we gently tightened the threaded plastic caps on our test machines…and forgot about them.

A couple of weeks later we preparing to port a pair of rave cylinders that had just been removed from a customer’s boat. After seeing that the guillotines did not retract all the way up, we noticed that the threaded caps were loose. This customer never noticed any unusual noises or poor performance, so it was impossible to know how long the caps had been loose. For curiosity’s sake, we checked the threaded caps of two other cylinder sets being prepared for porting…they were loose too! Then we started checking all the test boats we had in the shop…the caps on every single one was loose (even the one we had assembled and tightened two weeks earlier). As a result of this experience, we recommend that these plastic threaded caps be checked regularly to assure that they are tight. As an added measure to “slow down” this inevitable loosening, we also recommend to put a zip tie at the bottom lip of the rubber rave valve diaphragms.

1997 Ignition Boxes (11/16/96) – Sea Doo calls their cdi brain box an “MPEM module”. The 1995 XP 800 and the 1996 XP have MPEMs that are slightly different from one another. To accommodate this circuitry difference, the makers of aftermarket high rev modules for the laydown rave machines have been forced to make their rev modules specific to each of those two year models. For 1997, Bombardier has, once again, changed the circuitry in the MPEM units in the laydown rave machines. The folks at Micro Touch (who brought this situation to our attention) have already embarked on development of a rev module for the 1997 machines, however a problem looms for some 1996 owners as well. Apparently the last 1000 1996 GSX models were assembled with this new 1997 MPEM unit. That means that none of the existing aftermarket rev modules will work on these machines. Your Sea Doo dealer has a service bulletin (#96-34) that denotes all the affected GSX models.

Base Gasket II – After the above experience, we started to look very closely at the thicknesses of our base gaskets. The Sea Doo factory gaskets are supposed to have .1mm (.004″) of thickness for each hole stamped into the center pattern. However we have found many gaskets in our inventory that were too thick or too thin by .15mm (.006″). This may not be a big deal on a stock engine…but “we” certainly think it’s a big deal on our Sleeper and Hammer engine kits. In any case, we urge all rave owners to check their squish clearances, and measure the thickness of your base gaskets. What you see (in the hole pattern) may not be what you get.

Base Gasket Thickness – “Jon’s Story” – One of our early test boats was an XP that was to be raced in region one offshore. We shipped the kit to the owner, who had it assembled at a local shop. After the break-in, the test rider reported a little better top end, along with average bottom end power. We were concerned because all our other test riders had reported big increases in bottom end as well as peak rpm. After a few unsuccessful attempts to resolve the problem with carb tuning, he came to our shop with the machine. After a quick test ride, we found this XP to have mediocre acceleration and a 6900 rpm peak (that’s 150 rpm short of the norm). Back at the shop we checked out every possible problem. Besides the indicated compression being just a little low, the only other inconsistency was that the squish clearance seemed to be about .010″ (.25mm) too thick. At the time, we didn’t think this was the problem…but it was the only thing we saw that we could “fix”. We removed the .024″ (6 hole) base gasket and replaced it with a .016″ (4 hole) base gasket. The next morning we gave the boat a short break-in…then gassed it. The difference was unbelievable. The boat pulled viciously up to 7040 rpm…just like all our other test boats. No one was more shocked than us, that .008″ of base gasket thickness could turn a weakling into rocket. After this experience, we specified for our assembly instructions that a .038″ – .042″ squish clearance must be maintained on all Group K modified top ends. Since then, we have not experienced this problem again. (Note : This XP won the 90 minute 1200 pro overall at the Havasu Global Offshore Finals)

Fuel Consumption – Since offshore races are often won by quick fuel stops, we sought to get the maximum mileage from our GSXs. The design of the GSX fuel filler leaves a large air space locked in the top of the fuel tank. We found that by loosening the top clamp on the fuel pickup, we were able to purge out all the air, and gain about 1.5 gallons of capacity. This procedure should be done before the race, on a work stand, out of the water. The chaos of a pit stop does not make it time effective for a race stop. However the end result is a first tank that can last about 110 minutes (on a 7200 rpm GSX “Hammer”). This permitted our GSX to run the 90 minute events with “no” pit stop, and require only a quick “splash” of fuel for the longer events. (NOTE : This GSX won the 90 minute 785 pro overall, and the 2 hour 1200 pro overall at the Havasu Global Offshore Finals)

Oil Injection Pump Screws – Two screws in the top of the stock oil injection pump are responsible for holding on a tin plate that doubles as a “check valve cover / cable end holder”. One of our low hours GSX machines had these screws come loose and fall out. The result was oil spewing out of the top of the open pump (this of course resulted in a fried motor). We immediately checked the screws on our other two GSXs, they were loose too. We secured them with Loctite, and have had no problems since.

95 Rave Guillotine – The Rave valves on the 1995 XP 800 are different from the later style valves used on all 1996 rave engines. The main difference is the design of the shaft where it connects to the guillotine. The design of the earlier shafts makes them susceptible to breaking. When this shaft breaks, the rave guillotine drops down to crash into the piston (it’s not a pretty sight). We would urge any 1995 rave owners, who still have the original rave guillotines, to replace them with the 1996 style. 1995 valves have the number “350” cast onto the face…1996 valves have “352”.