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MG MGB Technical - ignition problem

Hell Everyone,

My 70B quit running while I was driving a couple hours ago. I won't have time to deal with it until tomorrow night, so I thought I would post the symptoms here and hope someone can help diagnose the problem.

The car quit running abruptly while I was driving. I attempted to restart, the starter turns the engine just fine, but the engine does not fire. When I looked under the hood, I discovered that the #1 spark plug wire had come loose. This must have happened right when the car quit because no cylinder was missing. There is no spark at this wire, or #2. I didn't check the others. I also didn't check the LT lead from the coil to the cap. I can here the fuel pumps "click", so I don't think it's a fuel problem.

Thoughts? I've never had to diagnose an ignition problem like this before, so I would greatly appreciate advice on how to start. Thanks in advance!

Greg
GKS Scott

Greg, Les Bengtson has a very good article on ignition troubleshooting.
http://www.custompistols.com/cars/articles/ignition.htm

Clifton
Clifton Gordon

Greg,

The 1st thing I'll change is the little rotor arm in the distributor. I faced the similar situation twice, outcomes were the arm shorting to the ground. Hope this help.

Ennio
Ennio Wong

No help now, but the first thing you should do when you lose power is look at the tach and the ignition warning light, and that is before stopping, knocking it out of gear, dipping the clutch or anything else i.e. the momentum of the car is still spinning the engine. A bit difficult I know if you stopped at a red light or something but still.

If the tach has dropped to nothing you have lost the primary ignition circuit. If the warning light is on this is because the ignition has lost voltage from the ignition switch. If the tach is registering normally then the problem is either HT or fuel.

Clip a timing light onto the coil lead and each plug lead and watch for flashing as you crank. If the coil lead flashes regurlarly and consistently but one or more plug leads don't then it is probably the rotor (all of them) or cap (none of them) breaking down.

If all plug leads fire regularly and consistently (and remember the coil lead will fire at four time the rate of any plug lead) then it's either timing or fuel, so while the light is on 1 or 4 just point it at the pulley and check it it is around 6-10 degrees BTDC or close to your static timing figure of you know what that is.

The following relates to SU fuel pumps, can't speak for after-market types. If timing is OK then remove a fuel delivery pipe from a carb - be careful, it *should* spurt so watch out for a hot exhaust. No spurt implies no fuel delivery i.e. pump, to confirm direct the pipe into a container and turn on the ignition. If the pump doesn't click there's your answer. If it does, reconnect and try again, and if it works not it could be an intermittent pump fault as you should definitely get a spurt from the fuel pipe just after switching the ignition off.

If you got the spurt and have one carb it could be a carb problem. If you have twin carbs then really the engine should continue to run - albeit poorly - on just one carb.

It could also be a massive vacuum leak, I've heard of the Welch plug being blown out of the ends of the inlet manifold pipe before now.
Paul Hunt2

BTW, if the fuel system is good you *shouldn't* hear an SU pump click if the engine isn't running. If you can either the suction-side non-return valve is stuck open, you have a massive fuel leak somwhere between the pump and the carb(s), or if the pump is chattering you are 'out of fuel'. This last could be down to a leak in the suction pipe, which could be the pick-up pipe indise the tank rotted through above fuel level.
Paul Hunt2

"BTW, if the fuel system is good you *shouldn't* hear an SU pump click if the engine isn't running. If you can either the suction-side non-return valve is stuck open, you have a massive fuel leak somwhere between the pump and the carb(s), or if the pump is chattering you are 'out of fuel'."

or you have an after market pump installed, such as one of the little, square, run all the time, make a lot of noise, Facet pumps. Cheers - Dave
David DuBois

Thanks everyone for your comments. The points and condenser went bad. After replacing these, she fires up mice and smooth...

GKS Scott

Thats "nice and smooth" not, "mice and smooth"! As far as I know there are no mice living in the engine compartment...
GKS Scott

I have just got a 78 b. Has not been run for 5 years.
Started up after replacement of the fuel pump/ plugs.
Will now start up and run lumpy unless revved all the time. Once its warning up it will run on 4 cyls but soon starts to cut out, splutter and misfire. Will re-start but sounds like a real dog. Any ideas. Just been mot'ed, vehicle good otherwise. It would a shame to let it go bad.
noel prosser

I would do a complete tune up and a cleaning or rebuilding of the carbs. If it sat five years with gas in the carbs they may be varnished up inside. The points may be oxidized from sitting and moisture in the distrubutor may be causing the cap to track internally.
J Heisenfeldt

Noel. I would second J's advise. Do a complete tune up on the engine, including setting the valve clearances. That makes for a good beginning point.

Second thing is modern fuel. It goes bad and you can smell that it has deteriorated when you remove the fuel tank cap and sniff it (carefully--not a good idea to do too much of such things). I would drain the tank and, at the very least, refill it with fresh fuel. Better is to drain the tank, remove it, and have it hot tanked to clean out any residue inside. I have found that the residue of old fuel is dissolved by fresh fuel and the residue is carried through the fuel system, gumming it up. This leads to all kinds of problems and once caused me to have to rebuild a carb three times until we understood the root cause of the problem and had the tank cleaned.

Lastly, you have, if the car is original, a Zenith-Stromberg carb on the car. This has a rubber diaphragm as part of it and this diaphragm can deteriorate over time. It might be worth replacing to see what effect it has on the running of the engine.

Les
Les Bengtson

Noel - Before completely redoing the carbs, have a look at the needles (one each carb). I don't know about fuel in the UK, but here in the States, there are so many additives that I've had a thin brown-red coating accumulate on the needle, effectively cutting off fuel from being atomized.

You can check by removing the filters, lifting the throttle while running and see if the caburization is happening cleanly, aliong with the color.

If you suspect the needle, remove the top dome of each carb and remove the sliding body casting that has the needle attached - do this very carefully. Cleaning the needle can be done via very fine sandpaper, but be very careful so as to not change the needle's surface - removing too much material (ie, ANY) will make the mixture more rich.

I found out about this procedure after a couple of winter storages in the last few years. It is one of those few "carb problems" that weren't electrical.

Aside to Les - don't forget, the UK cars were never "blest" with the Z-S carb - they stayed with dual SU's.
John Z

John. You are correct. Did not read the part about it being a UK car. Time seems to fly by faster than it used to.

Les
Les Bengtson

This thread was discussed between 12/05/2008 and 15/05/2008

MG MGB Technical index

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