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MG MGB Technical - My Gremlin has returned.

Ever since I rebuilt my engine about 6 years ago, my 67 BGT has this annoying habit of periodically just dying. No sputter, no gradual loss of power. The tack drops immediately to zero.

I pull over, move the coil wires, the leads to the distributor, get in and it starts right up. Sometimes it will do this a few times on a trip. Other times it is fine.

Turning the key on and off when it dies does nothing.

It has not done this for a number of months and tonight it started again.

It has to be electrical. My distributor was rebuilt by Advance Distributors at the time of the engine rebuild.

I have replaced the coil to distributor wire twice and cleaned the terminals.

Any thoughts?


Bruce Cunha

Any interruption from the ignition switch, through the tach pickup, coil and distributor to earth on the engine will cause the tach to suddenly drop to zero.

Next time it happens also look at the ignition warning light while the momentum of the car is still spinning the engine. If that is glowing (assuming it normally works) then that says power has been lost from the ignition switch. If it only comes on when the engine stops then the problem is closer to the coil and distributor.

Assuming you have points, or an under-cap electronic ignition system, the circuit includes a very flexible cloth insulated (originally) wire from the points plate to the distributor body. That is continually being flexed as the points plate twists back and fore under the effect of changing carb vacuum, and eventually fractures.

The other way to narrow it down is to temporarily connect a wire to the points terminal on the coil and run it back to a test lamp or voltmeter in the cabin. That should normally flickering or glowing dimly/reading a few volts while the engine is running. If it glows brightly/reads full voltage when the engine cuts out the problem lies onwards through the distributor. But if it goes out/drops to zero then the problem lies back through the coil and tach. Reposition the wire accordingly.
paulh4

Bruce, I concur with Paul on the ignition switch. I had the same thing with my 79 B. In fact, I am on my fourth switch. Same symptom each episode.

Cheers

Gary
79 MGB
gary hansen

Thanks Paul. An easy test to set up. The Ignition light does not come on when the tack drops to zero. It comes on when the car stops.

I will check the wires inside. I am running points.

I also think it could be the block that brings the feed into the distributor. I noted the mail terinal moves a bit.
Bruce Cunha

You remind me that the 25D4 with the spade terminal on the side has the same cloth-covered wire, and is also twisting back and fore so can fail in the same way.

You can see both of these cloth-covered wires (albeit black in this case) in the attached (ignore the black and white circles). There are a couple of suppliers of the earth wire in the UK, including Moss, Moss US are here https://mossmotors.com/ground-wire-replacement. Theirs have clear plastic insulation and may need 'some adaptation' (their words) to fit a particular distributor.

The supply wire is 37H2981 and more commonly available e.g. https://mossmotors.com/terminal-bush-lead.

45D4 distributors have the condenser wire going out through a hole in the body. This wire is plastic-insulated but has a greater number of finer strands of copper conductor which makes that more flexible than standard wiring, but they can develop internal fractures causing the same problem, a replacement condenser is needed for that. Earth wire as above and more likely to be a direct fit.


paulh4

I had a repro ignition switch in my MGA, the switch was only a year or two old but I began to get similar symptoms to yours.
The ignition would just switch off, all power to the switch was cut.

It turned out to be a faulty switch and I had to replace it with (hopefully) a better quality one.

I know that the B switches are different to the MGAs, but I would bet that some modern repro MGB switches can be just as badly made as my MGAs switch was.

Colyn
Colyn Firth

Doesn't sound like an ign. switch issue to me really---if wriggling the wires fixes it--
Something else to have a lookout for is, if you've had it apart ,just make sure the spade connector on the dist. and also the coil if it has spade connectors are fitted on correctly and nice and firm, I've found spade connectors that have been pushed on but with the spade inbetween the connector and it's insulated cover instead of into the connector itself and when the cover gets warm/soft contact with the post gets lost------
Also check the security of the little nut that holds the wiring onto the contact points in the dizzy and the integrity of the wiring inside there
Years back I had a 2Lt Escort that would cut out going round right hand corners when driven hard, chased it for a while and it ended up being a faulty ignition coil----?

willy
William Revit

It won't be the ignition switch itself if the ignition warning light doesn't come on until the engine stops spinning as reported.

Original coils had riveted spade terminals and these work loose such that wiring flapping around as the car moves will break the connection - BT, DT. Replacement coils should have threaded studs and nuts, but you have to be careful to get the right one as suppliers and even manufacturers information can be confusing. 12v 3 ohm for CB cars, 6v 1.5 ohm originally for RB cars as they had a ballast resistance in the harness. If that has been bypassed then they need a 12v coil, you have to check both car and coil to get the right combination.
paulh4

And confusingly some 1.5 ohm 6 volt coils are boxed and sold as 12 volt, to distinguish them from 6 volt equipped vehicles. (Ford Pop)?
Also I crimp and solder eyes on the coil wires and bolt them to the coil.
Upside down coil............gravity as well as wear, vibration and quality of electrical connection!!!
Allan Reeling

This thread was discussed between 23/10/2019 and 28/10/2019

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