Off the top of my head, here are some things that I have seen cause a muffler to glow when it probably shouldn't:
1. Old muffler with thinning/rusting metal.
2. Carb too lean. Causes the engine to run hot and without enough lubrication in the combustion chamber. If you are running lean the spark plug will be white.
3. Carb adjusted so rich that you get backfires. Explosions in the muffler will make it hot. If this is the case your spark plug will look black and foul.
4. Wrong spark plug and/or wrong gap. I don't know why this is, but I suppose it could be related to incomplete combustion.
An easy thing to try it to get out the manual and adjust the carburetor jet to factory. It should be easy to do, just screw it in all the way, then back out as much as the book says. I guess it depends how comfortable you are with working on your equipment though.