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MG MGB Technical - Coil oveheating

Since a few weeks the coil in my MGB 1962 is getting to warm. The cars runs very bad, no power in the engine and sometimes it stops and does not run at all. At that time the coil is hot but not so hot that you cannot touch ist. Most of the times the rotor is out of order. When i change that the the motor starts again but is stil running bad.In the exhaust i here explosions when i try to accelarate after a drive.
Yesterday i noticed that the needle of ampermeter was going a bit up and down. I connected the dynamo direct to the controlbox so the amperemete had no function anymore.From that time the engine was doing better and i could go faster and beter.
What can be the explanation for this.

Excuse me if my English is not perfect. I hope everybody understands what i am writing and mean to say.
B Bert

Check the dwell (gap) of your points. If they are too close it will cause the symptoms you have. Also check that the capacitor has not gone bad (check by installing a new one). Cheers - Dave

PS. - "Excuse me if my English is not perfect"

No excuse is necessary, your English is a whole lot better than my Dutch :-)
David DuBois

It sounds to me like you have either a bad distributor cap or faulty high tension leads. Try substituting these for known good ones and see if this improves the situation. This fault has nothing to do with the charging circuit.
Iain MacKintosh

If there are bad connections in the ammeter circuit it could well affect running and starting. It's one of the reasons why ammeters are a Bad Thing, a voltmeter would be better.

There could be many parts of the ignition low tension (LT) and high tension (HT) circuits that could cause this, and an ignition fault is indicated if you are getting backfiring in the exhaust.

Unfortunately being a 62 and having a rev counter you can't use the first diagnostic tool for bad running which is the electronic tach. However you can get an idea if the problem is in the LT or HT side by connecting a voltmeter between the points terminal of the coil (normally CB for an original positive earth car) and earth. Ordinarily with a running engine this should show about 4-5v (points closing and opening with a dwell of 60 degrees). If it starts jumping up or down from that when the misfire occurs you have an LT problem. If that stays steady then it indicates an HT problem.

Coils run pretty warm, but will be hotter than they should be if the points gap has closed up, and will overheat if the ignition is left on with the engine stopped while testing things.

Is the rotor visibly faulty when this happens? And if so in what way? Or is it simply that changing the rotor makes things better for a while?

PaulH Solihull

I agree the capacitor is the most likely culprit, the "made in china" ones we get these days I would say are "variable* quality and it could be going short circuit. Its possible that there are some shorted turns inside the coil, this will increase current drain and drastically reduce the EHT voltage. I got so tired of component failures that I put a Boyer Bransden
assisted ign on, this doesnt need a capacitior and the points should last 30K miles. I used them because I have kept my car +ve earth, even though they are aimed at motor cycles they work fine on MGBs.
Stan Best

B.Bert, Save yourself a lot of trouble, Buy the 123 ignition system which is made in Holland it is so good! Mike
J.M. Doust

Since this thread started there has been an article 'elsewhere' that claims the maximum resistance for an MGB 12v coil should be 3.4 ohms, and anything less than that is faulty and a cause of overheating (coils). This is quite wrong, 3.4 ohms is the *maximum* resistance for a standard MGB coil, the minimum is 3.1 ohms, and that is at 20C/68F, it will be even less than that in the sub-zero conditions we are experiencing at the moment. Also 12v Sport coils are typically 2.4 ohms and these are perfectly suitable for use without overheating. There are a number of other inaccuracies in the article, like calculations for current and wattage which are only valid when leaving the ignition on but the engine stopped, which *will* overheat the coil and should not be done, if you want to work on other ignition powered circuits then you should disconnect the coil first.
PaulH Solihull

The 12 Volts sports coil is a good idea.
After experiencing lots off problems with several standard coils, I fitted the 12 Volt sports coil.
Now I don't have any overheating problems off the coil since more then 2 Years.
For me and my 1976 mgb roadster it worked fine this way.
J Halma

The sport coil is *lower* resistance than the standard coil as I indicate above, hence passes more current and generates more heat than a standard coil. If you found standard coils were overheating where a sport doesn't, then either each of the standard coils you tried were faulty, or if they were 6v coils as befits a 76 then maybe your ballast resistance was short or partially so. Neither standard nor sport coils should overheat, if everything else is as it should be.

Incidentally I trust you *have* bypassed the ballast for the 12v sport coil or you will be getting weak sparking. Even then you will have disabled the starting boost feature which is the main reason for 6v coils and ballast on rubber bumper cars.
PaulH Solihull

As an aside to Paul's comments on the Sports Coils ~ They may get hot and hotter, but the one I currently use is about 14 yrs old on a daily driver. I mention this only as a reference point from one person's experience.
Robert Muenchausen

This thread was discussed between 22/11/2010 and 10/01/2011

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