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MG MGB Technical - Ignition Issues

Hi Everyone.

i have a 71 BGT, that has developed an ignition problem. The car was running rough and someone suggested changing the coil. I did, then one day (a week or so later)the car decided to splutter to a stop. I had it towed home and decided it check out the problem. I put the old coil back on and it ran and then stopped. Anyway I then decided to take off the electronic ignition, as this might be the fault, as it 5 years old. I have put the points back on but still can't get it started.

Does anyone know the correct set-up for the Low-Tension leads. I have white going to positive on the coil, white/black to negative on the coil and a lead that comes out of the side of the dizzy and to the negative on the coil.

I have noticed thats since putting on the points the tacho does not jump when i turn the car over, it did with the elec ignition fitted.

I don't want to keep changing things, without knowing whats happening, any tests i can do with a test light/voltmeter?

Cheers

Scott
Scott Ralph

Everything sounds correct, but I have no white/black to the coil on the negative side of my '71. Only the wire to the distributor. Could that be the oil pressure sending unit wire?

An ohm/voltmemeter should show no continuity from the points/condenser wire to ground when the points are open. It will show continuity when they are closed. Check for continuity from the points to the coil. Sometimes the wire breaks.

What condition are your cap, rotor, and plug wires in? Do you get any spark at the plugs or from the HT coil wire?
Jeff Schlemmer

Thanks Jeff,

i don't seem to be getting a spark from anywhere! the cap and rotor are nearly new. The leads a couple of years old.

I have put a test light on the wires attached to the coil and all seems well. Haven't tried opening and closing the points to see if anything changes.

I have three coils at the moment, the old one and two new, none seem to work, could the coil be the problem?

Feels like i'm beginning to run out of potiental problems! thing is i'm sure it's something simple, just need to work out what it is!

The tachometer is jumping when i crank the engine. i remembered it jumped when i started it before, as the engine cranked, is this significant?

Cheers

Scott
Scott Ralph

If you diconnect the HT coil lead from the distributor cap and you can't get that to spark to ground while cranking, then the problem is not in your distributor. If there is spark there, then its probably not the coil.

Disconnect the wires from the coil and check them for correct voltage and continuity. The tach should show a reading while cranking.

If you have an old cap and rotor, you may wish to try them. Is the center terminal (graphite piece) in the distributor cap still in place that contacts the top of the rotor? Sometimes they fall out.
Jeff Schlemmer

Check the rotor.

Sometimes the rotor will develope a short and
will ground through the distributor cam/spindle.

A high volt coil will exacerbate this.

My bet is that there was otherwise nothing wrong
with your original setup.
Daniel Wong

Scott - Have you checked the gap on the points, or better, the dwell angle? The dwell should be 60°, which corresponds to the proper gap. The other thing to check is the condenser (check it by swapping it out with a new one). Both an improper dwell/gap and a flakey condenser will caus poor running. Good luck - Dave
David DuBois

Hi Guys,

still no spark, i have changed the condensor, rotor arm, points and dist. cap. The engine turns fine, just doesn't want to fire.

The tacho doesn't jump when i turn the engine, is this significant, i always remember this jumping when the car tried to start.

I have checked the power from the white lead to the positive and there is power, i suppose i have to start looking at the low tension circuit. Any advice?

Cheers

Scott
Scott Ralph

Scott. Go to my website, www.custompistols.com/ and click on the MG section, then articles. There is an article on how to troubleshoot the ignition system which may be of use to you.

Les
Les Bengtson

Scott
Do you have the small twin nylon bushing seperating the wires on the points pivot post correct? It should isolate the condenser wire from the lead wire coming into the dizzy.
Mike
Mike

Acually, Mike, the nylon bushing serves to insulate the spring arm of the points from the post rising out of the base of the points. If that is not done, the power from the coil goes directly to ground through the base of the points, to the base plate to the distributor body. If the condensor were isolated from the input wire, the system would not work properly. Again, this is covered in my tech article along with a sequence to troubleshoot the low tension ignition system and determine what may be not working properly.

Les
Les Bengtson

As someone who got things out of sequence re: the nylong bushing in putting in points and couldn't get it to fire up, I would check out Les's info. Simple but critical. I generally make all the mistakes early on so I recognize them the second time around ;-)
J.T. Bamford

Hi guys,

i managed to get it going, seems the fibre washer was missing from the points, meaning the points weren't causing a spark when open

Thanks for all the help

Scott
Scott Ralph

What did you mean by the Fibre washer? I may also have a similar problem.
A MacNeil

I am not aware of any fibre washers used in the Lucas 25D4 distributor. None of my tune up kits come with one. The common problem for people who are installing points for the first time is to simply remove the nut on top of the points, attach the wire from the coil and wire from the condensor and try to start the car. This is how the system works on the older American cars, but not how the system works on the older Brit cars.

On the Brit cars, you have to remove the nut, remove the nylon spacer from the upper hole in the leaf spring section, install the wires onto the spacer, install the spacer onto the leaf spring so that the wires are touching the spring, then, install the nut.

The leaf spring section of the points set is insulated from the stud coming up from the base by a nylon spacer on the bottom and a nylon spacer on top. It is, thus, insulated from the base of the points set and the base plate of the dizzy. If the wires make contact with the base plate or points base, they simply short to ground and there is no make-break circuit to trigger the coil. Plus you have a short to ground which can burn up wires. (This can also happen if you are troubleshooting an electrical problem with the ignition switch on and the points happen to be closed at the time--melts the points, melts wires and adds several hours to your troubleshooting.)

Les
Les Bengtson

This thread was discussed between 09/04/2006 and 18/04/2006

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