Bill, it is important to identify whether you have a fine mesh screen across the muffler's entry port. That is a spark arrestor. It is required in some places in the US, and it is good practice to use one if you ever operate in forest or grassy areas, especially in summer. The spark arrestor is detachable. If you have any kind of screen or filter across the entry port, just remove it and try running the machine with the muffler fitted, but without the spark arrestor. If that fixes the problem (which it very often does) you then need to spend some time with solvent and a toothbrush, cleaning the screen until you can see through it easily.
If there is no spark arrestor screen, you most likely have a mass of something inside the muffler. There are two common types of mass. One is a wasp's nest, the other is a build-up of oily soot due to the machine having too much oil in its fuel for a sustained period. Solvents don't usually do much to wasp's nests, though they sometimes make them worse. My view is that the best and quickest fix for a mass in the muffler is to heat the whole thing up to a fairly bright red and hold it at that temperature for a while. If it smokes a lot, be encouraged: that shows you needed to do it. Then let it cool off naturally - don't quench it of course - and bang it at various angles on the bench, hoping a lot of loose stuff will fall out of the entry and exit ports.
