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MG Midget and Sprite Technical - Coil failure?

Had a bit of an odd one today. Went out for a run in the car (1275 ) today . Seemed a bit down on power and flat on acceleration and topped out about 3500 in top and was really flat . Got progressively worse . Stopped checked plugs etc, carbs fuel etc and it cooled down a bit . Set off home again and was better but soon deteriorated to the same eventually limped home and rigged up a spare old coil I had and it seemed fine . Not taken it for a proper run yet . Question is do coils do this , I thought they just fail ? Is it just a coincidence it’s better and something else amiss. Car has electric ign no points. Thanks
Mike Fairclough

Maybe you've fund the culprit but your symptoms are the same I experienced with a dodgy rotor arm, that was solved with a genuine red one from the Distributor Doctor.
Jeremy MkIII

Mike
I'd welcome a second opinion from someone who's experienced this but a coil could degrade with heat due to breaking down of insulation but still "work" with reduced HT voltage output. It may recover, as you've found, when it cools. But it may also be an intermittently failing rotor arm, as Jeremy says.

It does seem to be a heat related issue.
Bill Bretherton

From my limited experience, the symptoms you’ve described match a failing coil I’ve experienced in the past.

That’s not to say it isn’t the rotor or something else.
Philip Sellen

Lots of electrical components will deteriorate in performance when they are failing and get hot. I once had the symptoms you describe and while it was low output from the coil that was because the alternator had failed and I was driving at night on an increasingly weak battery (Mk 3 Ford Cortina).

If you have a known good rotor arm you could fit and test that before buying a new coil.
Daniel

Its why everyone who owns a Spridget, should own two of the spridgy things. Then whenever anything at all goes wrong you only have to swap parts between the two to trace the fault.
GuyW

Well thanks for all the comments, it seems it was the coil. Just did the same journey with the replacement “old” coil I kept on the shelf and all is fine . It does have a DD red rotor arm but I took an old one out with me just in case . Just for interest the failed coil was a not very old Accuspark item. The one on the shelf I had must be over 20 yrs old . Moral is don’t throw anything away and don’t assume new is better ! Great to have all your input thanks hopefully I’ll be Sprinting on Sunday 👍🏻
Mike Fairclough

coils certainly do this-
I notice you have electronic ign------how long have you had that and did the coil come with it
Why I ask is some suppliers seem to think you need low resistance coils with electronic ign. Which in itself 'can' be correct depending on which type of electronic ign. it is.
Most of the common aftermarket electronic ignitions are supposed to run with the std. coil for the car which for your midget would be a 3ohm coil. 'IF' by chance you've been supplied with a 1.5 or even a .75ohm coil, then it will overheat and breakdown with slow/normal driving
William Revit

Yes it’s a 3ohm coil that came with the ignition module from Accuspark which hasn’t given me much confidence with the module too.
Mike Fairclough

How is your coil mounted, in the normal place or elsewhere? There is a train of thought that they should be mounted vertically not on their side as the oil doesnt cover everything that way and so they overheat leading to failure. Having said that many placed on their side have lasted forever. Maybe newer coils are more prone to this than originals but I have no proof of that.

Trev
T Mason

It’s in the standard location Trev pointing upwards. I have seen some pointing down too
Mike Fairclough

I once had a coil overheat and blow the end insulated cap off and all the oil poured out. I was touring in Scotland and didn't have a spare so I stuffed the electrical bits back in, mounted the canister upright on end and topped it up with 20/50. It worked, and lasted fine for over 800 miles before I got home.
GuyW

Mike. The original coils were mounted nose downwards in the belief that this allowed them to, for some reason, work better. Something about the oil being at the same end as the centeral electrode was the standard reason given back then. I have found that when the coil is mounted nose downwards, many people over tighten the coil bracket, causing the outside case to deform--sometimes sufficient to allow internal shorts to develop. Some 50 years ago I began to mount the coils on my cars with the nose upwards and just sufficient tightening of the coil bracket around the coil to prevent upwards movement when pushed firmly on the bottom of the coil. My oldest running MG has been set up this way for something over 25 years now with no problems.

I, years ago, owned a Sun Engine Analyzer machine which allowed me to show the sparking voltage of each of the spark plugs. I never saw more than 18K volts required to cause the plugs to spark. The standard coil is capable of a maxium of 20K volts, more than I was ever able to record being required to cause the plug to spark. Hence, I have never considered one of the Sports Coils, or other "upgraded coils" to be of any measurable benefit to most MG owners.

Glad to see that you probably have the problem sorted.

Les
Les Bengtson

This thread was discussed between 28/07/2024 and 01/08/2024

MG Midget and Sprite Technical index

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