Briggs 128602 0304 Was given this self propelled craftsman push mower free. It has the primer style carb. It fires up second pull, blows white smoke. There is oil residue in the air filter area. Smells like burning oil coming out of the exhaust. Oil level is where it should be. It runs rough. Does this seem like major internal damage? Could this be something simple in the carb? Ideas please.