Check out this thread as it might help your problem.
http://www.seadooforum.com/showthread.php?t=915
and this thread on rave valves too:
RAVE valves
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Below is a quotation from the shop manual of the theory on how and why the RAVE (Rotax Adjustable Variable Exhaust) system, patented by Bombardier-Rotax, works on giving the high reving, high power capacity to the Rotax engines.
THEORY
For a 2-stroke-cycle engine to have high power capacity at high crankshaft speeds, a high volumetric or breathing efficiency is required and the fresh
charge losses must be minimized. The result is achieved by opening the exhaust port early and utilizing the resonant effects of the tuned exhaust
system to control fresh charge losses. When an engine of this design is run at a medium speed, efficiency falls off quickly. The relatively high exhaust port effectively shortens the useful power stroke and because the exhaust system is tuned for maximum power, there is a large increase of fresh charge losses. As a result, the torque decreases along with a dramatic increase
of the specific fuel consumption. Higher torque along with lower fuel consumption can be obtained at lower engine speeds if the time the exhaust
port is open, is shortened.
These guillotine type slide valves should be cleaned once a year. If your having a slow response or loss of power until the engine reaches a certain RPM and then all of a sudden, it's like your kicking in a passing gear, chances are, your RAVE is sticking open. These valves are very simple to clean.
To clean the RAVE valve, simply remove the black plastic cap, then the two allen head screws on each side. Be careful while pulling up on the RAVE so as to not break your gasket. The RAVE on the Magneto side will need to be seperated at the bellows (diaphragm) to remove it. Unless your prepared to remove your tuned exhaust system to get it out. Just remove the cap and use a metric sized socket to fit the bolted cap to seperated it from the slide valve. Don't worry, it won't fall into the piston. There a notch in it that keeps it from dropping onto your cylinder head. After removing the cap, simply pull out the slide valve.
To clean the RAVE valves, I use the spray type carb cleaner to remove all of the carbon deposits. If you use some type of metal to scrap with, remember, always use a metal that is softer than the metal your cleaning. This will keep you from gouging the valve. In my case, I had a piece of copper tubing, but found a small wedge of wood to be just as effective. Inspect the tapered edge looking for knicks or damage. This will let you know that at some point, your piston has hit the valve or a foreign object made it into your combustion chamber
Below are some pictures of the RAVE, complete and disassembled and cleaned.
Attached Thumbnails
http://www.seadooforum.com/attachment.php?attachmentid=45&d=1192810855
http://www.seadooforum.com/attachment.php?attachmentid=46&d=1192810876
http://www.seadooforum.com/attachment.php?attachmentid=47&d=1192810887
