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Discussion Starter · #1 ·
Hi all, I'm new to the snow mobile engine world. I've been toying around with single cylinder two-strokes for years. But I'm not familiar with the twins at all. I live in So. Cal. And snow is not that common. I bought this 85 ski-doo formula plus type 537 engine on line. This power plant is to be installed into an odyssey and I need to know the best light to mid performance mods to be done to the motor. The motor came with the stock 2 into 1 pipe; pair of 38mm round slide carburetors, all electronics- gauges wiring harness, etc… it also came with the stock driven clutch and a comet 102c drive unit. The bore is a standard bore and pistons/ heads are in good shape (no pitting, carbon build up, etc.) Any help would be greatly appreciated.
 

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Depends how far you want to go. Pump gas? Twin pipes? Stock these motors are around 95 hp. Full mods can get up to 140. If it's an 85 it should have 40's on it.
 

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Discussion Starter · #3 ·
I thought they looked larger than 38's. i would like to run 91 octane pump gas, twin pipes if i can fit them into the cage. my goal is to have a somewhat competetive racer but still keep the reliability. so way out porting isnt neccesary. my biggest thing is that i am not that familiar with the rotax engine and i am seeking advice before the work happens. All engines are costly when you screw them up. any and all information is helpfull. thanks
 

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If you want reliability, you can work on the porting. Exhaust polish, leave the transfer timing alone, but can add some volume. Work on porting and flowing the case. The rotary intake needs a lot of case volume to store the charge. Unlike the case reed (or piston port to a lesser extent) which needs a tight case to draw directly from the carb. If you are going to run summer only, you can take it to 14.5 to 15 to 1 compression, with .060 to .065 squish and still run pump. The single pipe works well on these motors. Bore the 40's to 42.5 and cut a valve to 145-75 and go from there.
 

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For your use I'd stick with the single, or try to find a FAST fat-boy for it. More torque with the single. Is the primary a TRA or the old three ramp roller? Is the housing orange? It would be preferable to run a TRA and a formula secondary. (94 or newer for cam availibility) and lose the comet driven.
 

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Discussion Starter · #7 ·
its a 102c drive clutch and the stock bombadier driven. how about enlarging the exhaust passage and polishing it. the vehicle is a light ride. about 600 lbs with driver. 14.5 - 15 to 1 compression. doesnt that equate to 195 to 200 psi? if so it sounds high for pump gas.
 

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PM me your email address & I'll send you the mods that Ski-doo recommended back then (1985)
 

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If you really want to make that motor work drop a line to Greg at Jaws performance,I think that this is the motor he used to use for watercrossing.Also Macdizzy in California could probably help you out and hes alot closer.
 
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