One very simple thing you can try is to just vent the tank out the bottom of the fuselage. Do not hook up vent to either muffler fitting. Most sport engines run just fine this way. You will probably need to adjust the mixtures a bit. You can tell how much pressure attaching the vent to your muffler produces by the size of the mixture change required to run without it. I have seen home made mufflers for scale planes with a "pressure" fitting installed that is actually causing suction.

Multiflyer