Peter,
The reason your pump self primes is that the oil level in your Belltank is higher than the pump therefore the oil fills your pump by gravity. This is not the case with the wet sump set up, when the pump has to lift the oil out of the sump. If you look at the pump internals the rotor runs a clearance of up to .006" for the Rover sintered rotor and .010" in the case of the QED steel rotor within the outer rotor, in view of this clearance the pump will not move air unless there is sufficient oil present to make a gas tight seal between the rotors.
Edited by - Rob walker on 3 May 2002 18:52:18