Glad you're running well now. It just goes to show you how the Bisson and Slimline Pitts muffler makers do not give any thought or attention to what the correct exhaust opening should be. It's a typical problem with the Pitts mufflers. Don't the makers realize that when they design a muffler with basically double the exhaust opening outlet area compared to stock factory muffler that the engine wont perform properly. They only shape and build the muffler but basically don't know, understand or appreciate the details of nitro engine operation. At least the info you needed was here to be found.