I have a 25cc Homelite converted engine that I have flown for a few years now. It has always run strong and been extremely reliable. A short time ago the engine has been unreliable returning to idle. When the throttle is cut the engine unwinds very slowly and the final idle RPM's will change each time the engine is cycled. I put a new carb on the engine and the linkage is a straight shot and functions properly each time. Any ideas what might be causing this problem. I took the engine to a local small engine repair shop but they were no help.