On an overhaul. Start it and let it idle till the radiators start to warm. Shut it off and let it cool. Do it again. Listen to the motor, smell it, and feel it. Any doubt, turn it off. Take it for a leisure ride after the last cooling, 5-10 minutes, let it cool. Then go for 15-20 minutes, let it cool. Then it should be good to open it up. It may sputter on top the first time you clear it out good. If it still breaks up on top, its still tight, or you have jetting issues. If the piston has a single line scored down the exhaust side, the bridge was too tight. If the ring/top of piston seized, you may have did it?