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MG MGB Technical - does not fire

I have a 1977 MGB that started to backfire and skip. I have replaced the points (previous owner installed a 25D distributer), rotor, cap, condenser, wires and plugs. Now it seems that I am not getting spark. Am I looking at a coil problem or am I missing something else?
FA Salvaggio

Hi, Could be coil or retarded timing or an induction air leak.

FA. Please go to my website and look at the articles under the MG section. One is an article on how to trouble shoot the ignition system. Website is and contains additional articles you may find useful in the future.

As to your problem now, the first thing I suspect is that you assembled the points incorrectly and have a direct short to ground. The base of the points is the ground point and the spring arm has two insulating plugs, one on top of the stud coming up and one on the bottom between the spring arm and the base. If the points wire is not installed correctly a short happens.

Correct installation is to remove the nut on top, remove the insulator plug, install the wire from the coil and the condenser with the plug through the holes, install the plug into the spring arm, then, install the nut on top of the insulator to hold the wires in place.

Removing the nut and installing the wires is a common mistake because that is how American points are installed. The Brits used a different model and most of us have made the mistake the first time we did a tune up.

If that is not your problem, check out the website for a through trouble shooting procedure. The other tech articles might help you to diagnose your other problems.

Les Bengtson

First thing is to clip an inductive pickup-type timing light onto the coil lead and each plug lead and check the light is flashing when you crank.

If it is flashing regularly (and the coil lead 4 times as fast as any plug lead) then the ignition is OK, so just check on 1 and 4 the timing mark shows about 10 degrees (not critical at this stage) BTDC.

If that's OK then after a bit of cranking take out a plug. If it is wet it is flooded. If dry and no fuel smell there is no fuel getting through. Should be a strong smell of fuel but not wet.

If not flashing at all, you need to do some dc tests on the coil with a voltmeter. With the points closed you should have 6v on the coil +ve and 0v (ground) on the coil -ve. With the points open you should have 12v on both coil +ve and -ve. If there's no voltage at all on the coil +ve there is a break back through the ballast resistance (contained within the harness) to the white or brown/white at the fusebox and ignition relay (if fitted). If the voltage is there but the same with the points open as closed, if its 6v and 0v the points are short-circuited. If its 12v the points or connection to them are open-circuit.

If that's OK it looks like a coil HT winding problem, although I've never heard of this winding failing.
Paul Hunt 2

Try the rotor arm. There are a lot of dodgy ones about at the moment. Make sure you have one with Made in UK on it.
R L Tinkler

check if there is power at the coil (+) and at the dizzy ( coil [-]) too. Check out the points. It is easy to install new points on a D25 that way that they are grounded throughout and would not work any more. Just remove the points and check carefully. May be a new capacitor should be fitted on this occasion too.

Also check the small black wire that is inside the D25, it is very flexible and isolated by fibres. Mesure it with a Multitester for function (check resistance) and free movement!

Fix a 12 Volts testing light between the negative (-) pole of the coil and ground. Crank the engine, if it does not light or switch on and off, there ia a problem with the points, not closing or allways closed.

There is no Whodoo within the ignition system of an MG B, just check it out in peace and you will cure the problem soon.


I agree with Les. I'd almost bet $$ on the likelyhood that the points are grounded. Been there....done that!!

BH Davis
BH Davis

Thank you, you were right. The points were grounded. I followed your instructions and she started right up. I still have some skip and backfire when I try to bring the RPMs up but at least now I have spark, I can work with that. Thanks again. This is a great forum.

FA Salvaggio

This thread was discussed between 29/05/2007 and 02/06/2007

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