I had the same type of problem on one of my planes last year, since it seemed to run through a tank of fuel a lot quicker than I thought it should. One of the things I was told was to make sure that the vent tube inside the tank was near the top of the tank, like GraupnerFan suggested. If it's not, then when you try to fill up your fuel tank, raw fuel will come back out through the vent line or muffler before your tank is completely full. For instance, if your vent tube inside the tank is 1/2" from the top of the tank, then your tank will only fill up to that height before the fuel that your pumping in starts coming back out through the vent line. So instead of getting 10 1/2 oz of fuel into your tank, you're probably only getting about 8 or 9. Hope this helps...Len