Way back, in the beginning, I tried the stock muffler and the Bisson pitts muffler. This was before any tweaking began on the carb. With the stock muffler, I could get it to richen up somewhat, enough to lose about 2000 rpm from peak. With the Bisson, it would not run rich, not even a little bit. By temporarily plugging one pipe with my chicken stick, it would richen up nicely at full throttle, and sounded better too. Before I flew the engine, I piched both outlets equally closed, to try to simulate the area of only one tube open. This is the way I flew for the first few flights, and this is when I had lean runs and dead sticks. I also had a sloppy transition and midrange. Partially blocking only one tube, I don't think, would provide enough pressure to the tank. The difference between the stock muffler and the stock Bisson muffler was only 200 rpm, so the ecxessive back pressure doesn't cost very much top end. Privided your carb is working acceptably, completely blocking one tube should get you the back pressure that you need. My carb never really ran acceptably, so I changed it to the Magnum carb. You may notce the rolled edges on the tubes of the Bisson muffler. This reduces the area somewhat. I had to cut my stack down for istallation purposaes, and I cut the rolled area off. Now the one is closed, and the other is wide open. It runs and sounds best that way.