you may be right piper, since the heli muffler has a much larger volumn and it make sense that when verticle, there is not enough pressure to push the fuel against gravity. But what I don't understand is that why it can pressurise a 330cc heli tank, but can't do so to a 8oz funstar tank. Could it be that my fuel tank(arf fuel tank) is too soft, resulting in the tank expanding instead of pushing the fuel when it is working against gravity? This coupled with the fact that the heli muffler gives less pressure results in no fuel feed to engine?