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Has anybody experienced this before and what was the root cause?

A normal 12v coil, non ballasted.
The wiring is running directly to the battery.
The car starts instantly and runs really well, however after say fifteen or twenty minutes the power suddenly drops away and the engine fades until a “ failing to proceed “ occurs.
The ignition coil at this point will be hot to the touch.

I’m carrying a spare coil and once swapped the car will start right up and run perfectly.

Until it happens again!

Any ideas??

How do you know its a normal 12v coil ?

Also do you mean that once substituted with the spare coil, it does the same thing ?

Measure the resistance of the coil - it should be approx 3 ohm

When you say "direct to battery" I assume it goes via the ignition switch?

Have you got points or electronic module? Have you checked the dwell?

The coils are the old Lucas type, made from the early 1930’s up into the 1950’s and the bases are embossed with model number, voltage and date stamp.

I’ve not checked anything else yet, the distributor is the pre-war type with shallow Bakelite base and no bob weights.

The feed is through and auxiliary switch, the car originally had a magneto and this is replaced with a “Coil Conversion”, a Lucas DC4 which sits on the mag platform and is a self contained unit with distributor and the coil sat on top.

If I swap the coil, the next time I run the car it does the same thing.

I am alternating between 3 coils…

I swapped the condenser, the points appear clean and are set correctly.

I’ve had a quick search online and one possibility appears to be the spark plugs, these are definitely pre-war, the car had been laid up since being bought for spares in 1954….

From the description, I assume this isn't a Mini or similar. Are you sure you don't have a mix of 6V & 12V parts? It's easy done when you are converting from Mag to Points.

Could likely be caused by a coil with wrong resistance.

I have known cases of coils marked 12v but designed to used with a ballast system - so really a 6v coil.

mk1 wrote: ↑Fri Jun 07, 2024 8:59 am From the description, I assume this isn't a Mini or similar. Are you sure you don't have a mix of 6V & 12V parts? It's easy done when you are converting from Mag to Points.

Rumbled!
It’s a car from the 1920s, but I’m aware of the collective knowledge within this group!
I’m pretty sure that the components are all 12v, but I will double check, I’m going to fit a new set of 18mm long reach spark plugs today and see how things go, I do have a couple of spare Coil Conversions, so if the problem persists I’ll swap the whole unit!
Thank You all!

Good luck. Please keep us posted.

Will do!

Thank You.

A silly question:
Is the low-tension side of the coil connected the correct way around...?

How old are your coils. Many old coils were swapped out and like most of us, the old one wasn’t thrown away. I had a similar problem on mk 4 mini. It would burn a set of points out in less than 5 minutes. New points new condensor stayed like that until I got a new coil.

andy1071 wrote: ↑Mon Jun 10, 2024 11:49 am A silly question:
Is the low-tension side of the coil connected the correct way around...?

Funny you should say that…
I checked it out this weekend and guess what?…
It was correctly wired, SW to switch and CB to contact breaker.

Bob46320 wrote: ↑Tue Jun 11, 2024 5:00 pm How old are your coils. Many old coils were swapped out and like most of us, the old one wasn’t thrown away. I had a similar problem on mk 4 mini. It would burn a set of points out in less than 5 minutes. New points new condensor stayed like that until I got a new coil.

Newest coil is 1956, oldest is 1931.
All have been running on cars, however the car does run hot, very hot in fact (model of car is notorious for this) and the coil kind of sits inside the radiator.
Next step is fresh coil, then a temporary re location away from heat source.

Just a thought here - do you have another vehicle you can try your coils in to see how they go ?

Is the engine earth strap in place ?

Or maybe spark plug gaps?…….

Vintage to Classic wrote: ↑Wed Jun 12, 2024 6:58 pm

andy1071 wrote: ↑Mon Jun 10, 2024 11:49 am A silly question:
Is the low-tension side of the coil connected the correct way around...?

Funny you should say that…
I checked it out this weekend and guess what?…
It was correctly wired, SW to switch and CB to contact breaker.

Is you car +ve or -ve earth do you know if that coil is from the same?

19 30's-50's coils would be I think be marked SW / CB = + earth, also I believe reverse polarity although not correct would be more liable to affect spark plug erosion than cause the coil to overheat, as previously mentioned check primary/secondary resistance and perhaps weigh a few coils try to establish oil quantity in them.

Can’t answer your question about the overheating but Austin 7s for example are negative earth some years and positive earth other years, I’ve got a Ford coil sitting on my bench marked up as power/distributor and it’s for a negative earth car.

Ok, so it’s a bit sparse on engine type and info, but let’s assume long stroke slow revving. So the coil is on longer than a fast revving engine for each spark cycle.
Perhaps an electronic ignition would shorten the contact time and reduce load on the coil??
Just an idea.