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MG MGB Technical - Fuel Problems

I have a 1975 MGB which was running great until I changed the oil and filter this weekend, and added Marvel Mystery oil to carbs. It stalled today and would not restart. Behaves as though it is not getting fuel. I discounted a fuel line at the carb and turned the ignition to on and the pump seems to be working fine. Filter is less than six months old. Any ideas? Thanks
Tim

Tim what do you mean you added Marvel mystry oil to the carbs? To the dashpots? I doubt changing the oil had anything to do with your car not running. Sounds as if it could be ignition. Have you tried a tune up?
chuck

Chuck:
I added the oil to the dashpots. COuld this have done something? As a test of the pump, I disconnected the fuel line near the filter, and could hear the pump clicking in the trunk when I turned the ignition, and fuel poured from the line. However, when I reinstall the fuel line, and turn the ignition, I am not hearing the familiar clicking from the trunk. I also removed the gas cap, and cleaned the ground wire on the fuel pump. The car was tuned in April. Any other suggestions? This is the first trouble I have had with this car since purchasing it in February 2001. Thanks.
Tim

Sometimes the needle and seat will get stuck closed. You might remove the float bowls and see if that is your problem. Sounds like you have plenty of fuel pressure.

Sean
Sean Brown

Tim,

I last came across Marvel Mystery Oil as a thin penetrating oil for freeing/lubricating turbocharger wastegate linkages on aeroengines. I think (and am willing to be proved wrong) that it will be much too thin for the dashpot dampers and will adversely affect the relationship between mixture enrichment and manifold depression. Try emptying them out and refilling with 20-50 as per the manual and seeing what happens.

Cheers,
Jim
Jim Aitken

Hi,
Yeah I agree,remove that thin oil from the dashpots. The thicker the oil (to a point) the better it will run (20/50). S U's are constant depression carbs.
The oil has a purpose: it is used to limit the amount that the piston in the carb rises (and admits more air) to any given amount of vacuum generated by the engine pistons sucking in a new charge for combustion. If the oil is thin the piston will rise too much and lean out the charge. E.G. If the piston rises too much it will act the opposite (negate) the choke and its effects When the oil in the dashpots is at the required level you will find acceleration is at its best. The dashpot piston is rising slowly as the rpms increase keeping the fuel mixture theoretically at its optimum. Nuff said. Alan
Alan

Thanks for the help. I have replaced the Marvel Mstery Oil with 20/50. I've also removed the air cleaners to check the carb pistons, and they seem fine.
When I turn the key to "on", the fuel pump is doing a very slow tap.(normally fast). If I remove a fuel line, it pumps steady. DO you think this is a weak fuel pump or carb problem? Thanks for all your comments. TIm
Tim Brazil

Tim,

You've checked fuel flow and decided it was good. When you replace the fuel line, wouldn't the pump tick slowly because the bowls are already full?

Does the motor fire at all? Because my guess is that while replacing the filter, you bumped the wires that go to the distributor or coil. Perhaps you have a weak connection that worsened as it was jostled in replacing the filter, and then engine vibration became the last straw.

I've heard it said that when you think you have a carb problem look everywhere else first. I'd take a quick check for spark.


Matt K.
Matt Kulka

This thread was discussed on 06/11/2001

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