I've never liked champion plugs in two strokes. Try a NGK B8es. Buy a couple of them. Also, Are you sure the exaust isn't restricted anywhere? Air should move freely through the silencer. I've also noticed stripped threads on the reed valve mounts on one of my motors. Be careful not to overtighten. If any are stripped, place a small washer and nylon lock nut on the back to make sure your Reed valve cage has a good seal.