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MG MGB Technical - stupid ignition question

This is probably really stupid - BUT - I have recently bought a BGT to replace my company car. The car runs fine except for a little miss at times, I thought I would replace the coil, dist. cap and leads - I did this but then found the car would cut out when warmed up over 2000 rpm, I replaced the old coil and it seemed ok so I presumed i had bought a duff coil. But then it started again !! the problem is the ignition is not standard so I have no idea how to check it. It has a Lucas Ignition Amplifier and what lookks like a normal dizzy but seems to have some sort of Lucas optical ignition system. As far as the wiring is concerned it all seems self contained except for the white off the coil which would appear to go to the tach and then ignition switch. Can anyone confirm the wiring for the Amplifier, it has 6 wires - 3 to the dizzy and 2 to the coil and an earth ? or any ideas ? dodgy cap and leads as well ??
Good old fashoined points seemed much more straight forward !!
Thanks - Chris
Chris Cordingley

Chris,

First of all, there are no stupid questions, only some pretty dumb answers! Mine may be one of them. If yours is a '77-79 model with the original Lucas 45DE ignition, you probably have the only operating one left in the entire galaxy. The amazement is not that it's failing, it's that it lasted this long! Perhaps you should report this to the Guinness Book of World Records, and then see if the Gaydon Museum wants to display it.

Your problem could be a number of different things, but even if yours is an after-market electronic ignition, these things do fail. I've had two Crane-Allison failures and one Pirana failure over the years. If yours is of unknown age/condition, given what you've already done (are the plugs OK?) I would replace it as a top priority.

Regarding the coil: is it possible that the new coil you bought is a 12-volt coil? The late B's ran 12 volts to the coil only while starting. Once started, the coil ran on 6 volts. I'm speculating here, but perhaps under operating circumstances, your coil was not getting it's rated input voltage so it flattened out over 2000 rpm. There is a resistance wire to bring the operating voltage down to 6 volts and that can be bypassed to give your coil 12 volts all the time, but it would seem that your old coil may be OK anyway.

' Just some random thoughts, FWIW,

Allen
Allen Bachelder

Thanks - it is a 1972 so it is aftermarket, the plugs look ok. I was considering the 123 ignition system as a replacement as I know nothing of the history of what is fitted. Apparently it had been converted to run on LPG and then converted back ? I must try and find out about it's past life.....thanks again Chris
Chris Cordingley

Chris,

Lucas produced two different electronic ignition systems.

One system called the Opus had the electronic module mounted directly on the distributor.

The other system called CEI had an external black box electronic module.

The Opus system was poorly designed and failed regularly. It has no servicable parts.

The Lucas CEI system is based on the General Motors HEI system which is a very reliable design and was fitted to millions of GM vehicles.

There is a small ignition module inside of the black box that can easily be replaced for around $35US. To order a replacement just ask for a four connector HEI ignition module. (The reason why you have to specify a four connector module is because the HEI could either have a four or six pin connector module.)

The NAPA part number for this module is "TP45SB Control Module".
S Rechter

Chris,

Here is a very informative link:

www.mgb-stuff.org.uk/ignitiontext.htm
S Rechter

I was considering 123 ignition, but decided instead to go with a recurved rebuilt dizzy for about a third the price. Here on this side of the pond, Jeff Schlemmer in Minnesota does a superb job. He's frequently on the MGB threads. If your car is a '72 model, there goes my theory about 6 and 12 volt coils.

But could it still be that your coil experience was coincidental with an intermittently failing EI?

So - I would suggest a rebuilt 25D, if not a 123, and equip the rebuilt with points and condenser until you discover if you've solved the problem. Actually, if you have an original 25D dizzy, just try points and get rid of the EI. Then if you want to go back to EI, try a unit like the pertronix. I've used Pertronix on several cars over the past 2-3 years and I have yet to have a failure.

FWIW,
Allen
Allen Bachelder

Probably an aftermarket ignition system as the UK never got the 45DE4 and 45DM4 ignition systems that North America got, and I'm assuming it is a UK car.

The only diagnosis you can do with these is replacement, preferably with points, initially at least.

For a 72 the standard ignition white wiring goes from the ignition switch, via the pickup on the tach, to the coil +ve so this sounds OK. However after-market ignition systems often cause problems with this type of tach, less so with the tachs that were used from 73 on.

Is the tach reading erratically with the misfire? Or steady? If erratic then it implies a problem with the LT side of things, although it isn't so clear-cut with electronic ignition as with points. If steady could be HT, which includes the ignition module output as well as coil, cap, rotor, leads and plugs, and could also be fuel. If you can reproduce this with your head under the bonnet clip a timing light (inductive pickup type) onto the coil lead and plug leads and watch the flashes. If the coil lead gets erratic with the misfire then it is the electronics or coil. If that is steady but one or more plug leads is erratic it is probably the cap or rotor, but do it in a darkened garage to see if the leads are breaking down anywhere. If on less than all then probably the cap, if on all the rotor or cap.
Paul Hunt 2

It's a UK car, the tach does go wild when it's misbehaving, I have put a new cap and leads on but will try and get some others to prove they aren't the problem. I have looked at the rotor arm and it seems to be lucas but combined with the optical "interferer!!" attached so as I am not sure what the system is I can't get a new one.. is it possible the tach can give problems ? is it ok to run the car with it bypassed ? it seems to be better with a lucas sports coil but you can still feel it's trying to splutter at times above 2000 rpm.
Thanks Chris
Chris Cordingley

You *could* bypass the tach by simply connecting the brown and white at the fusebox together, which turns on the ignition independantly of the key, and see what happens, but I very much doubt it is the tach causing the problem but the other way round. If it's a standard distributor I'd be surprised if the rotor arm was part of the electronic trigger, but I suppose manufacturers of after-market kit can do anything. If it *is* a standard distributor (should be a 25D4 41288 for a UK 72, although could have any reference number or be a 45D4 if swapped by a PO) you should be able to remove the existing trigger and retro-fit a standard rotor, points and condenser. If it is a non-standard distributor (there are various after-market types) you would need to contact the manufacturer, or swap it for one of the standard ones. *Any* 25D4 or 45D4 would probably be better than you have, even though the advance curves wouldn't be correct unless you got hold of a good 25D4 41288, setting the timing is a matter of advancing it just short of pinking at any combination of throttle opening, revs and load. Even if you get a rebuilt unit with 41288 stamped on the side there is no guarantee it will have the right curves, as I found to my cost.
Paul Hunt 2

This thread was discussed between 14/10/2006 and 16/10/2006

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