When you remove the restriction of the exhaust, it will lean the engine out. The carb will need to be retuned and richened up a bit. A GLOW engine will idle and transition better with a bit of muffler pressure because it helps keep the glow plug hot at an idle, but a gasoline engine does not need any back pressure from the muffler at all. Power can be increased by using a tuned pipe, or expansion chamber, but that is a different subject entirely. Some people think a gas, ignition engine needs some back pressure, but it is simply not true.

AV8TOR