nut 63 so that rear half 10d is rotatable with respect to the front half 10b thereby allowing exhaust tail pipes 21 to be directed in the desired direction when the fuel tank must be mounted very far away from the engine the variation in fuel head pressure to the engine will be much greater during in flight maneuvering therefore the fuel tank pressure must correspondingly be very high in order to make such variation in fuel head pressure neglibible such a high fuel pressure requires very narrow or restricted fuel passage through the carburetor 39 s needlevalve or fuel spray orifice narrow fuel passage is more prone to obstruction due to dust particle or gummy built up too small to be filtered out high fuel tank pressures increase the risk of fuel tank leakage in this situation it is better to use a fuel pump to suck the fuel up to the pump then feed it to the engine since fuel pump has no fuel metering ability a common practice is to pump fuel in continous circular loop to the carburetor and to return excess fuel back to the fuel tank without exerting any significant pump pressure to force the fuel into the carburetor the carburetor having one line for fuel inlet and another line for exit of excess fuel the carburetor venturi fuel draw will then aspirate the fuel into the engine but only the amount of fuel that it desires since a doubling of the flow rate of air into a fixed venturi carburetor will cause a four fold increase in fuel.