I have a 1980 Ski Doo Everest 500 E. Machine ran great for hours. When I start it back up again after a nice long ride, I have to give it full throtle to maintain normal idle. As soon as I release full throtle, the machine dies. I pulled the plugs whitch were brand new, and they were burnt(black). So I installed a spare set and she started right up with out a problem and this happens every year. What could be causing this? When I pull the one plug out, the cylinder smokes (if that means anything)Any suggestions would be helpful. Thanks.