There are many factors involved when using different brands of mufflers, other than the stock muffler. Many mufflers just do not provide enough pressure to the fuel tank to give adequate fuel flow. Many fuel tanks have inadequately-sized fuel line plumbing to allow for good fuel flow. Many people believe that the fuel inlet nipple on an engine is a problem.

Usually, we've not found problems with the large SuperTigre engines once you've gotten fuel to them. The inlet nipple is large enough for the carburetor's needs. The orfice at the needle valve will actually be smaller than that when the engine's properly-tuned.

If your engine's not getting good fuel draw, then you'll have to see what the difficulty may be. Many times, it's inadequate pressure to the tank. If that's the case, you'll need to find a way to increase pressure. Reducing the area of the exhaust outlet of the muffler is one possible solution.