Is the compression leaking out somewhere like the piston cyl at the exh, or just not snappy? You could take out a head shim to step up the compression a bit.Maybe a hotter plug? Also check to see if the plug is flush with the inside of the combustion chamber. A long plug may stick out too far or an OS type may be sunk in too much. It will make a difference with the little guys. It is always a gamble with used motors, I still keep getting them anyway. They are only $30 new, and parts are actually quite reasonable if you do need them though. Oh, I see Mr Cox beat me.