I had the same symptoms on a Saito .72. It ended up being the vent line inside the tank that was causing the problems. I used a bit of fuel tubing over the brass line in the tank as shown in the Dubro tank instructions. This was the first and last time I try that setup. Anyway, the section of fuel line was probably resting too firmly against the top of the tank and causing the tank to not feed without vent pressure from the muffler.