You dont' want to run muffler pressure with this engine. Muffler pressure will "supercharge" the engine and cause it to get too much fuel. It will backfire, throw props, quit running while flying for no reason and the high speed needle will be virtually impossible to set. Run a vent line from the top of the tank down below the fuse. When inverted, the vent line will still be above the tank and will not leak. Been running the sportsters ever since they came out like this and it is what they need.

Carl