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MG MGB Technical - Intermittent is an ugly word . . .

I have been trying to advise someone whose mechanical skills are even less than mine (we're very close to negative numbers).

His 1972 roadster's engine quits. After sitting for perhaps 15 minutes, it will start again and he goes merrily on his way. It may run well for days. One day, prepared with my procedures, he found fuel in the line and no spark. He installed a new coil.
The next time it appeared about to quit, he got it to catch by shifting to low gear and letting out the clutch, then pumping the accellerator to keep the rpms up. He says the engine is less likely to quit if he keeps the choke out 3/8".

When I have tinkered with his wires between failures, I can't get the engine to quit. As far as I can tell (at those times) his fuel flow is OK. He's afraid to take the car far from home.

The only good advice I have been able to render is to stay in the right hand lane, close to the shoulder.

We have completed the short trip to my wit's end --- any ideas?
Dan

Dan - The next time the car quits, try removing the gas cap. If you hear a "whoosh" when the cap is loosened, you will have found the problem. The fuel tank vent (in the case of the 72, it vents through the charcoal canister) can become clogged and cause a vacuum to form in the tank. When there is a high enough vacuum, the fuel pump cannot overcome it and the engine doesn't get any fuel. I have seen this situation in a friends car (a non MG) and on my 67 MGB while on a trip to California. I finally discovered that the vented cap was bad, but was on I5 somewhere on that desolate streatch between Bakersfield and Sacramento, so just left the cap off untill I got back to civilization and could get a new one. Good luck - Dave
David DuBois

Here's one thing I don't see mentioned much. When an engine dies from lack of fuel, it generally bucks a bit then sputters before it dies, as it uses up the last of the fuel. When an engine dies from lack of spark, it's sudden and complete. This may help you determine which system is failing on your friend's car.

I'd put in a see-through plastic fuel filter so it's an easy check for fuel starvation.

It's possible he's got dirt clogging a float valve. Enriching the mixture in the carbs might compensate enough to keep the engine running on one, albeit poorly. When it fails, you could knock the float bowls with a screwdriver handle and see if that makes a difference.

It's also possible the wire in the distributor has lost some insulation and is grounding, or is old and has an intermittant break. You can check the former visually. You can check the latter by getting the points to the fully open position, connect a test light between the moveable point and ground, turn on the ignition and then wiggle the wire.

Hope this helps.

Matt K.
formerly of Greenbrae, San Rafael, Larkspur, Terra Linda and Novato, CA.
Matt Kulka

I had a similar problem with my 72 roadster when I first got it. Turned out to be a bad groundwire on the battery.
william fox

I believe your car has the HIF SU's like my 73 B. After an engine swap, mine would run OK for a minute or two and then require the choke pulled out or else it sounded like a two cylinder. Consulting my Haynes manual it was clear that the choke initiates a separate fuel source so I deduced that maybe at least one main jet was blocked. I rebuilt both carbs with a kit and the engine now runs very nicely. The engine swap and carb rebuild were both firsts for me.

Good luck, Barry
Barry Quartermaine

Barry,
That is a new one for me. I never heard of a seperate fuel circuit for enrichment on an SU. Seperate air bleed on a few and even an accelerator pump on some Japanese SUs. How does this circuit work?
Leland Bradley

HIFs do have a seperate fuel enrichment device, it is not done by lowering the main jet like on HSs. It is a rotary valve with its own drilling that exits into the throat of the carb on the manifold side of the piston. The valve also has an air bleed. There is a third drilling in the same general area and that is the bypass port that comes up under the butterfly and stops fuel pooling in the throat at idle.
Paul Hunt

Does no-one suspect the lovely mechanical SU fuel pump? Sounds just like when mine went. Killing the ignition for a second or two then powering back on seemed to free the pump up. Also try hitting it with a spanner.

Or something :o)
--
Olly S
Oliver Stephenson

This thread was discussed on 15/05/2002

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