Hi Bill, I've come across a few trimmers like that and just recently a Kawasaki TH48 post hole digger with the same issue. Some mufflers run a screen(i think they call it a spark arrestor) which generally blocks up and restricts exhaust flow. If the FS120 has a spark arrestor clean it and it should come good. If not, before you rush out and buy a new muffler, try soaking the old one in a suitable solvent,eg carb cleaner, thinners, possibly even fuel for a few hours and blow it out with some compressed air and see if you can clear it up. When all else fails then go and buy the new muffler.
