+1. To Firepower's post.
In our club it has become standard procedure to plug one Pitts opening to get back idle.
If you look at the area of the two openings in the Pitts and compare to the stock muffler you will probably have the equivalent of double the siz opening of the stock muffler. The engine runs at the high throttle because it pumps up enough back pressure to the tank to flow fuel. At lower RPM there is not enough to pressure to supply fuel=no idle.
I cut small wooden or cork plugs to fit the opening, red RTV them in flush with the end, then drill a small hole in the side of the pipe to run a servo screw into the plug to secure. It is cheap and easily removable. Has worked on a couple of O.S. and Super Tigers with Pitts.
Good luck!