No question the exhaust gas temperature is higher when they are lean, Bob, and that heats up the valve and the muffler. A healthy muffler doesn't really have anything in it to burn unless it is on a 2 stroke, and there is probably no such thing as a healthy muffler on a mixed-fuel 2 stroke unless it's always run flat out. However we just don't know if Michael's ancient engine is healthy or not. He could have a gummed up carburetor resulting in lean mixture, he could have worn out rings leading to a carboned-up combustion chamber and muffler, or he could have gunk on his exhaust valve causing it to not seal properly.
