A good muffler will not overcome poor plumbing, improper tank locations, or a bad carb design. The S.T. carb has been a pain for ages so don't look at the muffler as a cause. It's quite possible the opposite is true and you now have too much back pressure.

Bisson mufflers are manufactured with reduced openings to assist those that need the extra back pressure. Those that don't cut off the piched outlet and pich up a couple hundred rpm. I've been using Bisson mufflers for years and prefer them over the Slimlines. Lower price and longer lifespans that Slimlimes.