Have a few of these but never this problem it is YS 120AC. Runs great puts out all the power no hesitation, run perfect except cannot shut the motor down after a flight. I believe it has an air leak some were but cannot find it. Stripped it down check everything cannot find a thing wrong. Replace all the silicone tubing. Have ordered a gasket set and an O ring set from YS-engines USA. But do not believe it will solve the problem. Even when I put my finger over the intake it is bearable able to keep running but it does for about 20 seconds, and then dies. Does anyone have any suggestions.