A two/four stroke engine with a ringed piston needs a slow, very rich break-in period to properly seat the ring. Typically I will run through a gallon of fuel on the bench with this type of engine. Some will say this is a waste of time/fuel...so be it. I'd bet you have 'dead-sticks' with a new engine you just 'put on the plane and run it', too. I NEVER have dead-sticks...period! A good bench break-in allows you to get the engine properly adjusted and tuned in order to run flawlessly once in the air.

Different strokes for different folks though.