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MG MGB Technical - Plug leads etc

I have a '70 B that will not start. It turns over freely and there is plenty of fuel to the carbs (bowls full). Cranked over with a plug out and earthed to the block and no spark. 12 volts to the exciter side of the new coil and the points work in that when open there is no ground to the coil and when they are closed, the coil is grounded. The HT lead from the coil to the carbon contact in the dissy cap is about 30k ohms (carbon lead) all the carbon plug leads have varying resistance according to length ( about 5 to 10k ohms). I am not sure if the resistance through the carbon leads should be this high. Can anyone help please. I have not discounted the condenser.
Also should there be a braided earth lead between the base plate and the dissy body?
Is there a check I can make on the new coil just in case it has failed?

many thanks Ian
I Denton

Ian - do you have high voltage from the coil to the distributor cap? If so, then you need to check the cap and the rotor. If not, you might want to check the HT lead connection at the coil end. Cheers - Dave
David DuBois

I as David suggested the next step is to check for spark at the coil wire, pull it out of the distributor and lay it close to ground [1/4"] watch the gap as you turn the motor over, if you get spark check the rotor no spark check the points an easy way is to leave the coil wire close to ground pop the d cap off rotate the motor until the points are closed then with the key on open and close the points with a screwdriver while you watch the gap, should be a little arc if not check the points. let us know RIC
RIC LLOYD

Ohms resistance in wiring is acceptable up to about 1K ohms per inch. However, that should only make the spark weak, not totally void of spark. If the condenser is weak you will see quite an electric flash across the points when opened by rotating the distributor with the ignition on or turning the engine over with the dizzy cap removed. Once again, makes it difficult to start, but not void of spark. I'm concerned about your next to last comment about the braided wire connector. Indeed, that would break the circuit. You mentioned the carbon contact inside the cap also. I assume you're referring to the carbon contact attached to a spring that inserts into the distributor cap. Make sure it's there and moves freely to make contact with the top of the distributor rotor. I've accidentally dropped the contact while taking the cap on and off only to suffer some consternation until I discovered why I had no spark.
Rick Penland

Thanks Gents, I am back to using your suggestions.
Rick, you mentioned wiring resistance is ok to about 1k ohms per inch, I assume thia is for carbon leads and not copper. I am using carbon, but on my TF I use copper. I know the theory that carbon is best for interference etc, and I guess that as the voltage is so high on the HV side, voltage loss would have minimal effect. As for the braided earth wire, I am assuming there should be one between the breaker plate and the dissy body, but I do not see it on the Moss diagrams, hence the question.
Regards Ian
I Denton

About 35 years ago when I was living in Venezuela, my good friend complained that his US Ford had lost most of its power. As it was a Company car about 2 years old,it was booked into the garage. He got it back after a multi Bolivar De-Coke but it was no better. I reluctantly offered to have a look for him.
The problem. During a Service they had fitted the Super New Type of HT Lead with a Carbon Core. The old copper was bent back to make contact. The carbon had no connection. A small fish hook in each end fixed it.
Cost - 7 Fish hooks and a can of beer.
Geoff F.
Geoff Farthing

Hi Ian,

If you have true carbon leads they would be well past their use by date. They are a series of fibres, like fibreglass, impregnated with carbon as the conductor. They were used to stop radio interference, which they did for a short time. I had a Morris 1100S which came with them fitted from new. After a few months the engine started misbehaving, and coming home late one night (probably out tom catting) I lifted the bonnet and beheld the aurora boreallis across the ignition leads. Chucked them out and fitted copper with separate interference suppressors. Worked great.

Today I would use silicon ones, or Bosch makes ones with a core of something with fine wire wound around it. Called Inductive core leads. They also seem to work well.

Herb
Herb Adler

Carbon core wires generate more heat to the core and tend to increase resistance with age quicker than copper, for example. The 1K ohms/inch rule applies to almost all types of wire that I know of with some performance wires advertised as generating 100 ohms/inch or less. They probably need to be replaced, but I'm not certain at what resistance it would lead to no spark at all and, hence that would only be part of the solution. I'm still curious about the braided wire in the distributor. You mentioned Moss doesn't show it, but do you have one in your dizzy? I just looked at Moss' site and they do show a "distributor base plate ground wire", part number 153-645. Moss also has some good videos and one on "Ignition Coil - How to Test". Just click on any distributor diagram on their site to see the available videos. You only mention a new coil and no other changes. Did the old coil allow it to start at least? If it did, I'd swap the coils and try to at least start it to eliminate or confirm that the coil is part of the problem. I drove a 70 for many years and recall when the condenser gets weak, the engine will crank for a long while slowly increasing rpms until it "catches" and starts. Have you turned the engine to the TDC mark, loosened the dizzy, turned on the ignition and rotated the dizzy to check for spark at the points and then at the number 1 plug? Also, have you checked the engine to frame ground strap for looseness. It's a braided strap on the driver's side engine mount on a 70.
Rick Penland

This thread was discussed between 01/02/2010 and 03/02/2010

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