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MG MGB Technical - Engine problem - electrical

I have a '73 MGB GT and it's been running great for months, but today the engine stopped suddenly as I was driving it, just as if the ignition had been turned off.

I can get a good spark from the coil HT lead to earth, but no spark from the leads to the plugs (they're new leads) which would suggest that it's something related to the distribution of the spark inside the cap. I've checked the distributor cap and rotor arm and all seems ok, no cracks etc.

The next thing I'll try is to replace the cap, but does anyone have any ideas as to what could cause this fault? Should I check anything else?

Cheers,
Matt
MattWilkinson

Matt,

a new cap and rotor arm would be on my list too, but i would also renew the capacitor within the dizzy, youst for peace of mind.

With the new wires, also check their sockets that are pluged to the coil and into the cap.

Hope this helps

Ralph

Ralph

Thanks Ralph. Yes, I was thinking of replacing the capacitor but isn't that on the LT side of things? As the spark is strong from the coil HT lead, would that suggest the LT electrics are working correctly?
MattWilkinson

I meant to add that I've tried replacing the coil too...
MattWilkinson

I know its obvious but have you checked the carbon contact inside the cap!

Carl
C Bintcliffe

Yes, I've checked the contact, it seems ok too.
MattWilkinson

Hi Matt,

I've the same situation twice, turn out is the little rotor arm shorting to the ground. Replacing it with new one, the engine came alive.

I searched the internet finding people saying that the current rotor arm's material is different from the past and the litter brass nail fixing the strip on the rotor is longer which create a shorter path to ground!

Ennio
Ennio Wong

Ennio has the right idea. Since you have a good spark at the coil, the points and condenser are fine. Just as Ennio has experienced a bad rotor with no outward signs of failure, I have had the same situation with a cap. I would suggest you try a new rotor and then a new cap if the rotor doesn't cure the problem. Cheers - Dave
David DuBois

I had once very odd problem with new dizzy, and as in your case, it was everything new, from coil, cables, spark plugs, and dizzy itself - at the end I found that occasionally capacitor lost contact to earth. It was impossible to see and it works fine at low RPM, but I got random problems during driving. When I took dizzy out and dismantle it completely I noticed that L profile that hold capacitor to base plate of dizzy fall away from capacitor (spotwelds simply broke).
toni

I'll throw my hat in with the bad rotor arm as
this has happened to me twice in the last
couple of years.

It's gotten me so paranoid that I know keep
a spare rotor arm in the boot.
Daniel Wong

Interesting to hear about the rotor arm problems, looks like I'll be picking up one of those along with a new cap. I'll give it a go tomorrow and let you know how I get on.

Thanks for all the advice.

Cheers,
Matt
Matt Wilkinson

Another thing I have noticed is the distributor is a 45D4, but in the Haynes manual it stipulates that a 25D4 was originally fitted (engine no. 18V582). Would this make any difference to how the engine runs? It was running fine before the fault but would it affect economy/performance? Thanks.
Matt Wilkinson

Hi All,

All my faulty arms only used for not more 3k miles and without any wear at the tip. I'm thinking is it possible to remove the little nail and fill the hole and fix the strip with epoxy? anybody out there try this method?

Ennio
Ennio Wong

Well I tried the rotor arm and it fired up straight away, now running perfectly.

Thanks very much for your help!
Matt Wilkinson

A consistently good spark from the coil lead but not from the plug leads is indeed pointing to the cap and/or rotor. Cliipmg a timing light onto the coil lead and each plug lead in turn should be the 1st diagnostic step in a non-starter, the 2nd in a car that dies on the road, the first being to look at the tach before you take it out of gear, dip the clutch, turn off the igntion or come to a halt. If the tach is still registering the problem is HT or fuel. If it isn't while the engine is still spinning then it is ignition LT. And if the ignition warning light is on as well as the tach having dropped, then the problem is the 12v supply via the ignition switch has been lost.

The 25D4 and 45D4 are fully interchangeable physically, but there are hundreds of different types of each with different curves, and running the 'wrong' curve will have an effect on performance and/or economy depending on how 'wrong' it is. For a UK 73 model it should be a 25D4 41288 (this number is usually underneath the installed distributor), what reference number is yours? My 73 also has a 45D4, and not even one for a later MGB. The big issue is that the original curves are probably of little relevance today anyway with the very different fuels we have. Unless you are going in for an expensive time on a rolling road, and recurving, simply adjust whatever you have so there is no pinking at any combination of throttle opening, revs and engine load with the fuel you regularly use. And paying more for the higher octane is definitely worth it as it allows you to run more advance and hence get better performance and economy and lower running temperatures.
Paul Hunt 2

Beware of carbon resistors in the rotor! Some replacement rotors for our cars have them as an Radio interference suppression method, and when the resistor breaks down, you get symptoms like yours. That is why I stick to non-resistor, all-metal conductor rotors.
Bob Muenchausen

This thread was discussed between 12/04/2008 and 13/04/2008

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