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Discussion Starter #1
hi, i have rotax 503 twin with one carb runs good , but if it idles it will foul clutch side plug.will foul that plug on warmer days. it has one coiland i rebuilt the fuel pump.cleaned carb anddont know why if fouls that plug with one carb and one coil. any help would be great, thanks
 

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My guess would be that the PTO cyl. is not up to snuff. Low compression? Bad crank seal? This side usually runs hotter than the MAG side and the piston may be partially burned.
 

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Discussion Starter #3
thanks for your reply, the pto side has 110lbs aND THE MAG HAS 105 LBS .ISPRAYED STRARTING FLUID NEAR PTO SEAL AND NOTHING HAPPENED.HAS ANY ONE HAD A COIL GO BAD ON ONE SIDE OF A COIL WITH TWO WIRES THANKS
 

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I don't think one side of the coil could go bad unless the wires look really dry rotted. It's only one coil and produces spark on both strokes to both cyls. It may be that your carbs need a synch?
 

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Swapped wires and plugs?
 

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I WOULD TRY A COIL, IF YOU HAD A SEAL GOING BAD THAT CYL. WOULD RUN LEAN AND NOT BE FOULING PLUGS.

GOOD LUCK!!!
 

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Discussion Starter #7
thanks alot for your replys theres only one carb i cleaned that . i cant switch the coil wires because mag side wont reach pto side. can the wires be unscrewed at the coil.i also adjusted the idle air about a half turn thisseems to keep the fouled plug dry but still black,and the other plug looks like it is a little to hot to me edges are a little white. thanks again!
 

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I know this is an old post, but did you ever figure out the problem? I have a 1979 blizzard with a 503 rotax and it is doing the exact same thing. I ran out of patience and bought a newer sled but would still like to get this one running.
 
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