Fuel pump loses prime

[Richard Michell]

My 48 TC has its "original" SU fuel pump, albeit with all new innards. It seems to pump well and has no trouble keeping up the fuel to the carbs.

However, if I don't drive the car for 24-36 hours it fails to pump fuel. Operates OK - in fact at very high speed - but is obviously dry and cannot develop enough suction to pull from the tank. Tank is typically only 20% full. Makes no difference if I open the filler. No sign of leaks in the line or at the pump.

My "fix" is to disconnect the discharge line, connect a hand bulb pump to the SU outlet, turn on the SU and assist it with the bulb. Once primed, the SU operates as it should.

Any suggestions please?

[Marv]

Maybe obvious questions, ..... have you checked the pre-filter screens at the outlet from the tank and also at the pump inlet..

Marv

[Steve Simmons]

Even the smallest air leak in the inlet line can cause problems. Or maybe an issue with the new diaphragm?

[Duncan M]

Richard- It does sound like it could be something to do with the diaphragm. Or "valves" below.

[Richard Michell]

Thanks all. Marv, I have blown the line back to the tank and checked the inlet filter numbers of times and I don't think that is the problem. Duncan, I have reset the diaphragm several times, using both the manual technique and the "power" technique. However I might try that again now that the diaphragm has had more use. It may be more flexible than when it was new.

I suspect the valves. They are new but, with their inherently very simple design, the sealing is potentially an issue. Not sure how to improve it but will have another look.

Thanks again.

[Marv]

Rich: Best of luck in your sleuthing, let us know what you find,

Marv TC5141

[Richard Michell]

Well, as tends to be the way, I thought that I had found the problem.

Careful observation revealed a weep at the union, under the car, between the rear fuel pipe from the tank and the front one to the pump. Tightening made it worse so drained the tank, pulled the union apart, found the solder holding one of the "teats" to the pipe had let go. Fixed that and also found the probable cause. The quality (if that is the word) of the brass that the replacement union is made of gives a very high friction as tightening proceeds. "Fixed" that problem by judicious use of low-friction thread compound.

Put it back together, put fuel in the tank, let sit overnight, no weep. Tried pump - still could not draw from the tank when pump dry. Primed it by the hand pump method. Now would only pump a trickle and sometimes not even that.

Pulled pump apart, cleaned the valves and the small amount of crud on the inlet filter. Lengthened stroke by one complete turn of armature (six holes), reset diaphragm, re-installed pump. Still would not draw when dry but , on priming, gave a very healthy stream.

Will road test and run for a few days to see if problem solved/reduced/unchanged/worsened.

[Gene Gillam]

Thanks for the update...good luck!