As you know, a catalytic muffler is designed to burn off excess unburnt fuel in the exhaust. A big problem occurs when you go to rich-en up the fuel mixture. The cat will get extremely hot and the edges of the hood will get burned from this excessive heat. This heat will shorten the life of your saw. The catalyst element itself is also a carcinogen: you don't want to ingest it when doing a muffler mod. A non-cat muffler is also noticeably lighter than the cat version.
