If the engine is able to draw air from other places then it will lean out and run hot. Seal the rear cover; replace the O-ring that seals the carburator to the crankcase; replace all of the other O-rings on both valves. Make sure there are no screws missing from anywhere. Use automotive silicon sealant on the muffler-to-crankcase flange and on the muffler halfs. Use Loctite on the screws holding the carburator in place to secure them AND seal them.