Some people, {{not me}} have had problems with the Pitts Muffler not producing enough pressure for the tank and they revert to plugging up one of the pipes. I have seen it said this cures the problem. When one of my .91s was surging I lowered the fuel tank and it cured the problem.
When I have a problem with any of my OS engines I go into the glow engine forum then to factory support and ask Bill Baxter what he thinks. He is my go to guy when I have an OS problem.