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MG MGB Technical - fuel pump mystery

In the last few weeks, my fuel pump (original SU pointed) has begun running all the time. What's not clear is where the fuel is going.

No overflow from the carbs, in fact, no flow through the fuel filter (transparent) when engine is off. Also no leakage under the car. Checked delivery rate and it's about 4 cups (32oz) per minute.

So why isn't the pump shutting off when the line is pressurized?

Thanks
Steve
Steve Aichele

Bad check valves in the pump?
Rob Edwards

Or, possibly, an air leak on the tank side of the pump. A new pipe from the tank to the pump is cheaper than a new pump, so might be worth a try first.

Good luck,
--
Olly
Oliver Stephenson

Steve - Disconnect the fuel line from one of the carburetors and dip it into a container containing some fuel. Turn on the ignition and watch the container, if you see bubbles coming out of the line then you have an air leak in the line between the tank and the pump or in the pump itself. It is then just a matter of finding where the leak is. The other possibility is a piece of dirt trapped under one of the check valves or a ruptured check valve as Rob suggests. For further information on the SU fuel pumps, see my article at:
http://www.custompistols.com/cars/articles/dd_su_fuel_pumps_101.htm
Good luck - Dave
David DuBois

I don't have any problems with poor running at high RPM, which led me to discount the air leak alternative. But maybe it's just a small leak...

I have had some trouble with faltering when accelerating from idle, but I thought that was related to a 30 degree F drop in outside temperature and no modification to the mixture.

I may not be understanding the wonder of SU pumps, but wouldn't a partially obstructed check valve result in decreased fuel delivery? If the 'in' valve were obstructed, on the push cycle of the diaphragm the gas would be pushed both toward the engine and back to the tank. Similarly, if the 'out' valve were obstructed, the pull cycle would draw gas from both the tank and the engine line. Either way, there would be substantially reduced gas delivery to the carbs, right?

In a related question, how does the pump know when to stop? The lines to the carbs get pressurized, does the pump continue to try to run, and only succeed when the pressure in the line is low enough? Or does is there some sort of presure switch?

Thanks
Steve
Steve Aichele

IMHO, it sounds like leaky In-Out valves.

They are a "reed" design and the works depend
greatly upon a thin, fragile, piece of plastic.

When the valves begin to leak the pump keeps
clicking away because it cannot build up enough
hydraulic pressure in the carb feed line in order to
cause it to stop.

Some of the fuel whooshes back & forth in the
lines, while some of the fuel is actually being
pumped into the carbs.

At first, fuel delivery volume is still enough to
maintain an idle and moderate speeds.

As the valves gradually get worse, the engine will
begin to starve for fuel when it is under higher demand. It gradually worsens as the valves wear
out and crack more and more.

Dismantle the pump chamber from the coil body;
remove each valve and examine them with a
strong magnifying glass. Look for any hairline
splits or cracks in the plastic reed.

Victoria British sells the valves. Moss doesn't.

While you're there, this would be a good time to
check the points gap.

Daniel Wong

Steve - Daniel has described the pump action when there are leaking check valves quite well. The reason that you can have bad valves or an air leak that keeps the pump from shutting off and still have sufficient fuel to keep the engine running without problems is that the fuel pump will pump much more fuel than the engine needs if there are no restrictions to the pumping action, like the float bowls filling and shutting the flow off. The pump stops pumping when the float bowls fill up because the pressure in the system builds up to equal the tension supplied by the volute spring in the pump. Quite simpley, the spring cannot push any fuel past the needle valve untill the float drops a bit, at which point the pump will click once or twice to build up system pressure again. Fot the AUF 300/AZX 1300 series pumps for the MGBs, the pressure is maximum of 2.7 PSI and a deliver rate of 18 gallons per hour or 2.4 pints per minute. Cheers - Dave
David DuBois

This thread was discussed between 17/10/2003 and 18/10/2003

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