Here's a little data for anyone trying to compare the Thunder Tiger, and the Evolution. These are my numbers with a dial caliper. Your readings may vary. The take away is the difference. The Thunder Tiger muffler exit has a taper to it, but at it's smallest it measures approximately .240". The Evolution is .315". The carburetors compare at .295" for the TT, and .300" for the Evo. Fuel feed to the Evo would probably benefit from a smaller muffler exit hole. Someone mentioned using the TT Muffler on the Evo. This would probably result in little on no loss of power, with greatly improved fuel flow. Now I have a question for the folks who have experience with the Evolution. The remote needle valve on my engine would not back out far enough to get a "rich" setting. I tried all kinds of adjustments with no success. Finally I removed the needle valve from the remote bracket and installed it in the traditional location on the carburetor, and it worked great. Has anyone experienced this problem? Greg