Priming the SU pump on SS and Mark IV

1938 SS saloon, SU Type L AUA-25 fuel pump
On this model and the Mark IV the pump is quite a bit higher than the tank, unlike the Mark V. The gas has a tendency to drain slowly down back into the tank.
It has been twice now, last year and this month, that after the car sitting unused for several months, that the pump won’t pump any gas up from the tank. It needs priming.
Last year I tried filling the whole fuel line from the discharge side of the pump and it took forever, fighting against the check valves inside the pump.
This time I disconnected the outlet line and removed both check valves and lapped and polished the inlet on 1500 grit sandpaper, just the bottom side. It is a brass disc 1/2" diameter and .018" thick. Stick it to a bit of masking tape as a handle if you want to try this.
I filled just the pump from the inlet side using a little funnel and a little hose that fit into the inlet fitting. I left the outlet line disconnected, so I could see when the pump was full. Reconnected the inlet and outlet lines. Switched on, and it worked immediately, pump clicked slower than before when it was empty, slowed down after a few seconds and stopped, system full ready to go. Cranked about 4 seconds and it started, and its 40F here today. Ready for summer!

If these pumps fail to draw fuel from the tank I usually suspect that one of the unions in the line from tank to pump is not totally tight such that the pump is drawing air from the said union.


Yes, quite right, and it would also leak fuel there.


Or suck on the carburettor side!!:nauseated_face: