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MG MGB Technical - 1980 mgb engine cutting out

I have a 1980 mgb that runs great except at interstate
speeds (70 mph). After running fine, the engine begins to cut out and loose power. When you depress the clutch it clears up and will run ok for a minute or 2 and then cuts out again (this can be done over and over untill reaching an exit ramp, but it hardly makes for a stress free trip) Anyone have a suggestion on what might be causing it, and what I can do to correct it? Thanks, Mike
m.j. manly

Hi Mike,
I have just posted a similar problem with my 1971 MGB on the MGB Technical forum, as a suggestion, you may wish to post your problem on that forum Regards Ian
I Denton


Mine has started doing this recently. Maybe once every hour on our motorways when the traffic thins!

Three other things to check:

Carb icing. If the weather is cold and humid, or if there is water in the fuel.

Fuel starvation, if your fuel pump is getting tired, faulty feed or earth, the fuel filter blocked, or any combination of gunge getting stuck somewhere.

The fuel tank filler cap or breather is blocked, not letting air into the tank and leading to low pressure that the pump can't fight against. This is my favorite since I know my tank sucks in air when I stop to fill her up.

David 78BGT
D Balkwill

Gentlemen - See the article, Fuel Delivery Troubleshooting Guide in the SU Fuel Pump Articles section of my web site at: for tips on finding fuel delivery problems. Keep in mind that often SU fuel problems can be traced to Lucas ignition components or adjustments.

David B. - "This is my favorite since I know my tank sucks in air when I stop to fill her up."

Try driving with the filler cap removed and see if the problem goes away. Your car's fuel tank should vent through the charcoal canister. There is a union in the line between the vapor separator in the boot and the engine compartment that often rusts shut, causing a venting problem in the later MGBs. Cheers - Dave
David DuBois

David DuB,

The UK spec cars aren't the same as yours. No charcoal cannister. The tank is vented via the filler cap which leaks air somehow, although
I can't see where. Anyway, mine now leaks air since I have cut a slot in the seal.

I'm driving south to get a ferry to France tomorrow, so I'll let you know how it goes.

David B.
D Balkwill

"This is my favorite since I know my tank sucks in air when I stop to fill her up."

On cars without the charcoal canister and the correct vented filler cap this is *normal* to a certain extent. The cap vent consists of a spring-loaded valve that needs a small amount of vacuum to have developed in the tank to open it and start letting in air. This stops fuel leaking out of a plain hole in a roll-over, as well as fumes on a warm day. If you have been driving a while and stop to fill up you should hear a slight gasp as you remove the cap which releases the remainder of the vacuum.
Paul Hunt

This thread was discussed between 08/07/2009 and 16/07/2009

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