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MG MGB Technical - Coil Overheating

Hello everyone.

Today, I experienced the misfortune of an engine cutout at fairly high speed (80 mph). Unfortunately, as I was more concerned with getting the car out of the fast lane and off onto a very fortuitously placed slip road, I failed to notice what the tachometer and eejit light were doing at the time.

I have read a few archives and I can see that without the knowledge these two bits of info would give me, I really have to look at checking all my connections, including the starter/solenoid, LT circuit, HT circuit, fuel pump (in fact, everywhere can't hurt).

However, I can't help thinking that the fact I was hairing along at high speed and had been for some while is not unconnected. Also, the fact that after about 5 minutes bu the side of the road, the car started up fine and I was off again. I didn't take it back up to those speeds, in the interest of getting home but the car was fine up to 60 or so.

Could this be the signs of a coil on it's way out? (or "off this mortal coil"). If so, would a direct replacement be recommended or are there any "sports"-type replacements which would give me better performance as well as costing me more?

Thanks for any hints and advice. I know, on balance I get more from this forum than I put back but there are occasions where I can be useful, I promise.


- Background -

1972 roadster - generally the car is if fairly good nick. I have recently replaced the needles with 6's to match the K&Ns, which involved tuning the carbs. The colour of the plugs is good (unless I have been sitting in London traffic too long) and I set the ignition timing at 10 degrees BTDC at 100 RPM the other day (dizzy clamps are tight). The fuel pump works well and is the noisy uprated, cylindrical type. Transparent fuel filter/regulator is fitted. I did not hear any vacuum in the tank when I removed the cap.

Regulators aren't always accurate, try a little higher setting.

Clifton Gordon

Was your transparent fuel filter full of fuel when it cut out?
Dave O'Neill 2

I think you're right, it certainly does sound like a coil to me, although there's no way to prove it until it happens again. Perhaps a wire is a little loose under the bonnet, maybe the fuel pump overheats, perhaps your ignition key jiggled out of the 'on' position, perhaps alien space rays shut down your electrical system in a failed abduction attempt, you never know what happened if it only happens once. If it feels like an abrupt and total shutdown and it happens every time you sustain high speed then my bet's on the coil, exactly as you say.

If it's random, then I'd look at the fusebox, then the wiring to the coil, then the ignition switch. Then I'd start cussing. Then I'd look into the fuel pump, and then I'd start watching the sky for strange craft hovering about.

As far as uprated replacements: if your fuel is already igniting properly and at the right time, then the ignition is doing great and an upgrade buys, well, not so much. True, automakers have upped their coils' voltages over the years. Higher voltage can handle a wider range of conditions and let you run a wider plug gap which buys a small improvement in efficiency. That's all. If I were you, I'd buy a sensible mid-range coil, higher voltage perhaps but not the crazy racing one, and if you want to splurge, splurge elsewhere.
Sam Good

Thanks for your thoughts guys. The fuel filter never seems to fill completely, rather it sits around a level around a third full. Maybe this needs adjusting? However, it never seems to empty either, which I guess is a good sign. To be honest, I didn't know you could adjust these regulators so I will do some homework.

Sam, do you really think I should cuss? It's just my mum always told me...

Until you mentioned it I had also ruled out aliens, but now I'm not so sure.

I suppose I really have to check al my wiring and see if it happens again (and then check all the symptoms).

Thanks for the advice regarding coils. I'm just interested to know whether some coils, as well as giving stronger sparks, are also more tolerant to heat?

Thanks again folks.


Alec, I should have said fuel pressure regulator. All I have seen have a knob for adjusting pressure. I had similar problems to yours on a hot day several years ago, it was caused by the fuel pressure regulator. I removed the regulator, no problem since removing it. Don't worry about the filter not being full. The amount of fuel in the filter doesn't seem to affect how my car runs.

Clifton Gordon

Original coils on that era had rivetted spades and these work loose over time, later they were changed to threaded studs and nuts. Exactly the same happened briefly a few times on my 73 roadster, original coil, although in that case I did see the tach at zero and the ignition light still off, which was what led me right to the coil/points area.

If it had been the fuel pump, a) the engine usually gets rough and stumbles before it cuts out altogether i.e. as the level in the float chambers is dropping, and b) if it is the type that stops when the float chambers are full e.g. standard SU when they are working again and you turn on the ignition there is much chattering while they refil the float chambers.
Paul Hunt 2

I had a very similar problem recently - engine would die after 20-30 miles at highway speed, would re-start and die again some miles later. Problem was found to be flooding carburetors. Alec, check the float height in your carbs.

Modern oil-filled coils are allegedly more heat resistant than the old ones, but then, Any non-faulty coil will survive bouts of high speed driving. My stock coil sustained 80 mph for two hours straight on a 100-degree summer's day in California. I nearly died from the heat but the coil was fine, by virtue of its non-faultiness. The only coil that will cause you misery, is a faulty one.

So, in purchasing your new coil, ask for one that isn't faulty. Insist on it. Preferably, go to a place that specialises in selling non-faulty parts. You'll be glad you did.
Sam Good

I knew I had to be going wrong somewhere. But is it possible to find someone who deals in non-faulty Lucas goods? Maybe in the same section of the yellow pages as toilet-trained bears. Hmmmmmm....

I will check the float chambers as you say, Willie. Maybe check the mixture again too as it does foul still in traffic.

Paul, the car did not stutter or stumble before cutting out. All of a sudden there was no power, just gradually decreasing forward motion.

A general checkover is in order and as I have never replaced the coil myself yet, a new coil can't hurt either.

Thanks again,


My rivetted spade original coil is still working well. I would change the condensor first, they can fail with heat, and are a lot cheaper than a coil.
Stan Best

Ironically the title of this subject hasn't been discussed yet. Also ironically, it wasn't long after I fitted the new coil that I mistakenly left the ignition on with the engine stoped for maybe 20 mins or more, and found the coil very hot and the engine wouldn't start. Smothered it in wet rags to cool it down and fortunately it started OK soon after. Since then I've done it at least once more, with the same results. Either I never left the ignition on with the old coil in a dozen years or more, or the new coil has some kind of thermal cut-out.

If the carbs are flooding I'd expect petrol to be pouring out of the overflow, which is much easier to spot on a UK car than a US with charcoal cannister. It also wouldn't explain a sudden and complete cut-out, and subsequent restart, that points much more to ignition.

Even cheaper than changing the condenser just in case is wait till it happens again, and this time look at the tach and warning light :o)
Paul Hunt 2

Thanks again guys.

Stan, the condesnsor is less than a year old but obviously that is no guarantee. Good suggestion.

I do occasionally get the odd whiff of petrol while driving but to be honest, I can't remember a time when I didn't. That is probably due to a slight leak around the petrol gauge sender as I have sometimes seen wetness there.

Paul, that is interesting what you say about your coil cutting out until it had cooled down. The more I think about this and the more I read, the more likely it seems.

Ah yes. If the points happen to be closed when you leave the ignition on, power runs through the system constantly - no surprise that it overheats! If the points are open, nothing happens. Pure chance. So to get away with it one day but not another is, well, normal. Check that you haven't burned the points at the same time.
Sam Good

This thread was discussed between 01/07/2007 and 06/07/2007

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