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MG MGB Technical - Fuel Pump Question

Went out today, after some 2 weeks of inactivity with the car ('73 mgb roadster) for a morning drive for coffee, bagels and the NYTimes.

Car started up fine, drove to town - 3 mis.; shut down, started up again, no problems, drove the other way - 5 mis. shut down, and then...

Car would not start, tried unsuccesfully for 2 or 3 attempts and then noticed the fuel pump kept on "clicking" after 5 secs. or so. Pumped gas pedal and car started fine.

Drove home and started to think, bad idea!

Gas line freeze? ( Car in garage, outside air tempt 22F) Put in some "dry gas"; tank 1/2 full. Had breakfast, tried to start again, but had to pump pedal and pump kept on clicking during start. Filled tank.

Car started and then drove on H'way.. No problems.

However, whenever I stopped, idle would gradually decrease below 7-800 RPM, normal 9-1100 RPM. Had to keep pedal pressure on.

Got home and let the idle go down; car stalled. Tried to get started again with gas pedal pumping, no deal; I think because of the slight incline of the driveway.

Pump keeps on "clicking."

Pushed car into garage.

Now I'm thinking??

Diaphgram? Air? New pump?

TIA
Joseph Lopinto

Joeseph. The normal operation of the factory SU fuel pump is to click until the system is pressurized, then stop. After that, the fuel pump will operate as needed, but you will not hear it because of the engine running. On my cars this is four to five "clicks" until the system is pressurized and the car is ready to start. The fact that your car continues to click indicates either the fuel pump is bad (bad diaphram) or the system is not pressurizing properly. The latter can be because there is an air leak in the line going to the pump or the pick up tube is broken. (In the latter case, the pump "runs wild" with a constant clicking as when you run out of gas. This latter is not from personal experience. I had a friend who borrowed my MGA to take his girl friend out one time. MGA did not have a functional gas gauge. He ran out of gas on the date and this is what he described. I think the young lady, whom he later married, was reassured by the fact that he immediately began cursing and went for the gas can in the trunk. Fortunately, for true love, there was a gas station within sight.) In any event, the first thing I would check is connections for tightness, the second thing I would check would be the fuel pump and the third thing I would check would be the fuel pick up in the tank. Assuming that your gas gauge works. Les
Les Bengtson

Sounds like a problem I recently had with my 74 SU HIF carbs. It would die or
run on two cylinders and then clear up and run ok. Turned out the Grose Jet had
the ball stick in the closed position. You should disconnect the fuel line at the
carbs and using a bottle or suitable containe rcheck to see if fuel is coming from
the pump. I forget the volume, I think about a pint per minute is enough. If the fuel
flow is ok then the problem is in the carbs.

To continue on the Grose Jet, the HIF carbs require a small tip Grose Jet because
the older style Grose Jet will bind the float. I looked at my spare set of HIF's yesterday
to install them on my 68 GT, one float in the HS4's on it has fuel in it and sunk.
I found one bad Grose Jet in the spares and the other one is intermittent. I had
planned to start a thread to ask if anyone has experienced problems with Grose
Jets in HIF carbs. Clifton
Clifton Gordon

Had diner, went out to garage, started car with gas cap off.

Pump kept on clicking. Fuel filter indicates about 1/2 full. No fuel leaks on garage floor. Full tank of gas.

Foot off gas pedal, engine slows down and stops.

Put cap back on, same result.

But why would car run fine on hwy. at 60mph+. No stutering. Only at a stop or idle.

I'm thinking that an air leak on the suction side of pump is culprit. Now to get under car in garage tom.

Where's that "milkhouse" heater?
Joseph Lopinto

Besides a bad diaphragm or an air leak in the intake line, it could also be the valves in the fuel pump.

The fuel pump valves are made of an "X"shaped, paper-thin plastic, and tend to develope cracks and
tears when wearing out. When that happens, in the beginning, there will be a drop in pressure and
the pump will click a bit more than it should. Later as things get worse, the fuel will just wash in & out of
the pump chamber and the pump will click continuously, but there'll be little or no fuel being delivered
to the carbs.

Victoria British sells new valves.
Daniel Wong

Joseph - Does your MGB have the original SU carburetors or does it have a Weber? If you have the SU carbs, pumping the gas pedal should make no difference in the car starting since the SU carbs don't have an accelerator pump, all you are doing is opening and closing the throttle plate. With the other symptoms that you are describing (idle RPM droping off), I would suspect that one of your carbs has a needle valve that is not shutting off the gas to the carb. The excess gas is either going into the charcoal canister or into the intake manifold. If the excess gas is going into the intake manifold, this would explain the drop in idle RPM (excess gas causing the mixture to go too rich). The same thing (rich mixture) could be why pumping the gas pedal is causing the car to start (if you have the SU carbs). Opening and closing the throttle plate could be admitting enough fresh air to lean the mixture out and allow the engine to start. If you have a Weber carb, the info about pumping the gas pedal does not apply. Good luck - Dave
David DuBois

A NA 73 with all emissions kit and a bad float valve won't dump fuel on the floor for a while, at any rate, not until the canisters have filled up. Remove the hoses from the carb overflows and turn on the ignition (don't start the engine). If the pump clicks and fuel comes out of an overflow then the float valve is bad.
Paul Hunt

My mgb roadster 1973 has been off the road for the last four year so decided to start her again i have replaced many parts i bought a second hand fuel pump on ebay su electronic fuel pump. I believe the bottom vent tube goes into the boot not sure about the top vent tube, where does that attach, many thanks clive.
c . r ayres

Both into the boot, there should be two holes in a 73, see attached. Can't remember which is which though!


PaulH Solihull

The vent spigot at the base of the coil housing on the pump is the only one that needs to be routed into a dry environment, such as the boot. The spigot on the end cover has a check valve in it that prevents air from entering that area - it is an exhaust only. Cheers - Dave
David DuBois

have not started my mgb in 3 and a half years i put two gallons of fresh petrol in the tank on top of any stale petrol that might have been left in the tank will i do any damage if i try and start it' i have given the car a full service and changed many parts all advice welcome many thanks clive
c . r ayres

"The vent spigot at the base of the coil housing on the pump is the only one that needs to be routed into a dry environment, such as the boot. The spigot on the end cover has a check valve in it that prevents air from entering that area - it is an exhaust only. "

Maybe, but they *were* both taken into the boot originally.

"i put two gallons of fresh petrol in the tank on top of any stale petrol"

It will be well mixed by now. You won't do any damage, but the pump may not start and the carbs may be gummed, unless either or both have received attention.

With the pump it will almost certainly have oxydised contacts, you will have to remove the plastic cover and clean the faces carefully with fine emery or wet and dry, don't force the contacts wide apart. In the past I have got away with simply bridging the contacts with a screwdriver so the solenoid operates a few times, and that has been enough to get it going.
PaulH Solihull

This thread was discussed between 01/01/2002 and 28/01/2012

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