It sounds like the high speed (main) needle is simply too lean. Try turning it out a few turns, don't be shy with it. Once you've got it running wide open, close the needle until the RPM peaks, then back off a bit.

I suggest leaving the glow driver on in the even it is way too rich to begin with.

I'd back out the low speed needle to start with as well. Once you have the high speed sorted out, you can turn your attention to the low speed.

I don't think the muffler would have much to do with it as long as it is about the right size, and supplying at least some pressure to the fuel tank.