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Here's a common one. Easy to do. Just cut a slit in the side of the muffler with a dremel and heat the area around it with a propane torch and bend into shape with a large slot screwdriver.
Take the diffuser out of the muffler and drill the 1/4" holes out to 5/16" or 3/8". Make sure you clean all the metal dust out of the muffler so it doesn't end up in the cylinder.
I find backing the area you are going to pry on is best backed up with a piece of hardwood to prevent distorting the side of the muffler body. This mod copies the muffler outlet on a Poulan 4000.