Had somewhat the same thing happen to a Bolens twin only it was one of the four screws holding the breather on the carburetor. Engine was running fine then all of a sudden I lost a cylinder. Pulled the head and one of the the four screws had lodged in the exhaust valve between the valve seats. The top of the head had a nice dimpled texture to it, clean as a whistle too LOL. Pulled the bolt out, put a new head gasket on and all was good. The four screws on the breather I retapped to the next size to clean up the thread and used Loctite, never had a problem..................Mike