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MG MGB Technical - 1975 MGBGT fuel problem...

Hi, I have a 1975 MGBGT. Recently (last week) I went out for a run and it broke down - due to the fuel pump which stopped working.

I bought a MG owners club electric fuel pump and fitted it. The old mechanical pump had seen better days.

I also replaced the fuel lines with braded steel ones ala MG owners club.

It started after a few turns and I took it to a mg specialist garage to have the carbs tuned (the engine was running very high revs)The engine ran fine after this tune up.

The car continued to run and then - it broke down again (yesterday).

I called the AA who did some tests:

1. The fuel pump keeps clicking - is this right?

2. When the old fuel pump was removed there was what appeared to be a breather tube attached to it - there was no where to attach this to the new fuel pump so it was left hanging. When the filler cap was removed there was the sound of a vacuum seal being broken. Is this right?

3. The AA man seemed to think the fuel filler gauge is also faulty as when he put a gallon in the car ran albeit it seemed very down on power (the gauge was showing half full). Is there a way to check the guage?

4. He also stated it might be best to replace the fuel filter (within the engine bay) as when he checked the flow rate of fuel to the carbs was very slow - Have I done something wrong?

Hope you can help


E McGee

Edd - You don't say if the replacement pump is a OEM SU pump or an after market pump. If it is a SU pump, there should be a vent spigot at the lower flange of the coil housing that should be connected to the vent line that goes into the trunk. This vent line will not affect the car's running, but will prevent any moisture from entering behind the diaphragm in the pump and will prevent rust from forming in that area. See the article, SU Fuel Pump Vent in the SU Fuel Pump Articles section of my web site at:

The fact that the fuel tank makes a whoshing sound when the filler cap is removed indicates that the tank is not being vented. The problem is either the vented filler cap (if used on your car) or the vent line gong to the charcoal canister in the engine compartment if your car has that system. If the latter, there is a union fitting in the ridged line going from the vapor separator in the left rear wheel well to the charcoal canister. This fitting will be accessible from the boot and may very well be rusted shut internally, causing the problem. Cheers - Dave
David DuBois

Dave, a UK car would not have the charcoal canister system and will rely entirely on the fuel cap venting the tank.

Edd, as you draw petrol out of the tank, air must be able to get in to replace it or the tank will collapse. The only place air can get in is through the fuel cap which MUST be a vented type. You get an inrush of air when you take off the cap. It sounds as though you either don't have a vented type, or the vent is blocked. What happens then is that the engine starts and runs fine, but then the pump has to work harder and harder to draw the petrol against the increasing vacuum in the tank. Eventually it just can't pump any more and so you break down. The pump keeps trying which is why you can hear it clicking. Normally the pump clicks until the carbs are full when back pressure switches it off.

Throw the cap away and buy a definitely vented cap. I bet it will be fine. David has told you about the vent lines.
Mike Howlett

Oh, and the fuel gauges are notoriously inaccurate. Just get used to what it tells you. Oh, and AA men don't really understand these old cars any more, unless they are near retiring age!
Mike Howlett

Hi Dave and Mike,

Thanks for coming back to me.

I spoke to the MG owners club techincal section this morning and they confirmed exactly what you said.

It seems the pipe attached to the old pump was to enable the diaphram on the pump to breath - it did not vent the tank at all.

They mentioned that the filler cap must have its vent hole blocked - well that will teach me to get a cheap one off e bay....

I have ordered an new one from the mg spares club.

I am also going to change the fuel filter and pipes in the engine bay as they look as if they have been on the car since 1975!

I'll let you know how I get on..


E McGee

Firstly, the standard vented cap has a spring-loaded valve which only opens when removal of fuel has caused a slight vacuum to occur in the tank, and this *will* make a noise as you remove the cap.

Secondly, if the cap *were* the wrong one and not vented, then it would have been doing this all along and not suddenly started. The only way that could happen is if you have replaced the filler cap as well, which you don't mention, or the vent had somehow just got blocked. Possible, but unlikely.

Thirdly, if it *was* excessive vacuum in the tank, then that would stop the fuel pump clicking, and its that which gives the fuel starvation. If the pump is still clicking then it must be delivering fuel - unless the pump itself is faulty.

And fourthly I'm wondering if you have bought the cheaper Moprod pump, which has a black plastic body, and no ports for the (two) SU breather pipes to be attached to. If so then these pumps have a habit of short-stroking from time to time, which means they click but do not deliver enough fuel. Been there, done that, slung the pump to the back of a drawer.
PaulH Solihull

This thread was discussed between 31/07/2011 and 02/08/2011

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