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I was looking at the microfich for a 1997 Ski-doo Formula 500 Deluxe - and it shows different sized main jets for the PTO - 310 and MAG-290 main jets. Does this have to do with the fact that this is a Rotax engine with rotary intake valves ?
 

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Yep. My STX takes different mains for the mag and pto carbs, as well...
 

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completely normal...not to do with intake valves..

It has to do with coolant. Some motors, the coolant runs through one cylinder, then through the other, then to the exchangers. the 2nd cylinder in the loop will always run a little warmer, hence need a slightly leaner jet (or is it richer?).

-Dean
 

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cooling's the answer...also found on watercraft...
 
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