AV8TOR, how much did you open up the muffler on your "free flowing stock muffler"? Did you just drill a couple of holes in it or what? I have a quarter inch hole drilled in mine and it increased the power a bit and increased the noise a little also. I have a 13.5mm carb coming in the mail and I want to make sure I have the muffler opened enough to realize the potential of the larger carb. However, I don't want it to be too loud as our flying field has neighbors. Thanks.