Ride on mower muffler smoking and turning red.

Hello All

So I have a poulan pro 42 inch ride on with a Briggs and Stratton 33r887-0009 engine. Initially I had a problem with with a stripped spark plug. I tried to use rethread but there was a hairline crack which it made worse. I decided instead of trying to further repair the hole that I’d just replace the cylinder head with a used one I bought on eBay. After much cleaning I installed the cylinder head with a new head gasket and torqued it down to spec. I set the valves both at .004. I also replaced the carburetor. And a new spark plug. I did an oil change and it was time to fire it up. The engine started and seems to run well. I was letting it run and adjusting the idle on the carb when I realized the muffler was bright cherry read. It was only running a few minutes. I immediately turned it off.

Any suggestion would be much appreciated.