erizzi,

The bisson is actually worse for back pressure. Something to try is to get some steel wool and stuff it into one of the pipes on the muffler. This will increase back pressure and not block exhaust flow too much. I found a long time ago on an OS160 is yes you can totally block one tube off but the engine suffers since it cannot breath as well. I then cut the tubes and stuffed one with the steel wool and then engine ran much better as it was getting the back pressure and could breath better. This was when using the Bisson muffler