Most likely, your engine's just not getting enough fuel. The muffler's probably not providing enough pressure to the tank. This is indicated by the fact that the engine runs very well with the stock muffler.

The other types of exhaust systems you tried may have been too restrictive, which is why you got the overheating tendency. You may have to use a less restrictive exhaust system and then start to block the exhaust exits to bring the pressure to the tank up to where the engine will run with the needle at 1-3/4 to 2 turns out.

We're sorry we don't have a certain answer for this.