I have a 1986 Fourtrax 250, it starts and runs great, idles nicely until it gets warmed up. After I have run for a few minutes it won't idle down, and the engine speeds up considerably. I have adjusted the air screw and idle stop disassembled and cleaned it but it still won't idle. The throttle return spring is fine, the butterfly and piston are closing properly. I'm out of ideas. Any thoughts other than replacing the carb? A friend has a carb I can have cheap but it is a little different. There is no mechanical linkage for the vacuum piston, I'm guessing it is vacuum operated and the throttle cable attaching screw is smaller on the carb. There is also a plunger on the side that I'm not sure what purpose it serves and it would be inaccessible when intalled.