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MG MGB Technical - Need troubleshoot advice - '79B

My '79B completely dies while running along perfectly. If it sits a few minutes, while I'm fiddling with wires under the dash and in the engine compartment, it starts back up and runs a few miles longer then dies again. I don't know of any wire that I "fixed" while fiddling. WHen it dies, all gauges go to zero, and it completely shuts down. When I try to start it the engine turns over fine but doesn't fire at all, ie., no coughing or sputtering at all. The clear fuel filter is full of gas. This sounds like more ignition than fuel to me. I had driven ~ 15 miles when this first occurred, then only about 2-3 miles before re-occurance. I think I have the original "Opus" distributor but I'm not sure - photos attached. Please provide some hints on things to check.


Tom Gillett

Check that the ignition switch -what happens if you just wiggle the key
check the low tension at the coil / distributor is not loose, and good and connections are good and tight all the way through the circuit back to the ignition switch.
Check nothing is causing an intermittent short anywhere on the ignition circuit.
Have you changed the condenser recently? If so, dig out the old one and refit it.

The saying is most SU carb problems are electrical, and if it otherwise runs fine the ignition circuit is a good place to start
dominic clancy

Tom - the symptons are aso consistent with a fuelsupply problem - the supply being slowed by a blocked filter, constriction of the pipework or a malfunctioning pump that slowly fills up the carb tanks while you are stopped but can't satisfy demands of a running engine.

Disconnect the fuel line with the ignition on - be prepared to catch a spurt - if it runs slowly then work backwards along the fuel line until you get to the pump.
At the risk of painting an unnecessarily black picture - don't stop at the pump - there could be a prob in the suction line - inside the tank even.

Good luck

R
Roger W

That the gauges etc die at the same time definitely points to electrical. The ignition switches do die, and it sounds very much like that it is the only thing in common with the engine running and all the gauges. You could confirm by trying your wipers next time it dies - if the wipers stop at the same time and come back on when you wiggle things, it would be good confirmation. Worth confirming as the switches aren't cheap.
Paul Walbran

It must be a miracle, but you still have the original Opus ignition system in your car. Almost all of these units failed early on and were replaced under warranty by BL with the '80 CEI ignition system. I agree that it sounds like a fault in the electrical system based on your postings. RAY
rjm RAY

Tom. It sounds like a problem in the white wire system, which includes an ignition relay which, as it begins to die, gives the symptoms you describe. But, if you still have the original internals on the Opus system (my daughter's car had the dizzy with aftermarket internals) that would be the first thing to check out. Get a copy of the wiring diagram and trace out the white wire circuit to find where the problem is.

Not a fuel problem as the primary cause--the tach winds down as the engine dies if it is fuel system related. But, from memory, the fuel pump is on the white wire circuit so, depending on where the problem is, fuel supply may be a secondary issue.

Les
Les Bengtson

I also agree with electrical (but not the ignition system) if the gauges die at the same time. From the schematics available North American spec cars with the ignition relay i.e. 77 on have the ignition warning light, fuel pump, OD and TCSA supplied direct off the ignition switch, everything else off the relay. The ignition warning light may give a useful diagnostic as to whether it is the switch or the relay if it glows when the engine dies or not. If it glows while the engine is still spinning then you have lost the 12v supply from the ignition switch, if it doesn't then it is the relay.

However the ignition light may only glow dimly for reasons I won't go into, whereas on other models and years it will glow brightly under the same circumstances. It also needs to have the original warning light wiring, i.e. someone hasn't inserted a diode in its circuit to prevent the problem where you can't turn the engine off if the emissions plumbing has been interfered with or is faulty.

If you leave the ignition on when this happens, and hear the fuel pump click which will normally happen within a couple of minutes of the engine stopping, then that also tells you the ignition switch is OK. You will need to watch the fuel and temp gauges at the same time to make sure they haven'y come back by the time the pump clicks, as that would indicate the fault has cleared itself anyway.

If you have OD then switch that on and engage 4th gear, as that should also make the light glow brightly.
Paul Hunt

I accidently clicked on mgb instead of a and saw your post. I was the original owner of a 79 and mine went out in around 83. By then no dealer support. When it went it would stall and then start right back up but as time went on it took longer before it would restart. I was told that as time went on someday it wouldn't start back up at all. John Twist advised me that the next time it happened to watch the tach,if it still had a reading it wasn't the module. If the tach stopped completely then get a new distributor. I had my A at the time so when the next time it happened and sure enough the tach stopped,I put the distributor in from the A until I could get another.The B was after all my daily driver. Your saying all your gauges stop throws me but I'd be surprised if after 35 years yours was ok. It will be interesting on what you find,good luck.
gary starr

This thread was discussed between 17/10/2014 and 22/10/2014

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