I actually "tune" my pitts mufflers by CAREFULLY crimping down both outlets while the engine is running full throttle until a slight decrease in rpm is detected, then open them back up with a screwdriver just enough to get the rpm's back. This has always given me a solid idle and performance that is at least on par with a stock muffler. As a plus, there is no risk of a plug EVER coming out. You will also need to retune as you do this, as the back pressure will change as you crimp, obviously.

For what it's worth-