I have had many strange things cause this. One was the cap on the carb for that cylinder was loose. Another that comes to mind is the needle and seat or float level. If the choke was stuck open it wouldnt get better when warm.if it was stuck closed it wouldnt foul the plug. IMHO.
A forum community dedicated to snowmobile owners and enthusiasts. Come join the discussion about performance, racing, troubleshooting, trails, maintenance, riding tips, modifications, classifieds, gear, and more!