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MG MGB Technical - Hot cut-out

It's an outside temperature thing, not an engine temp thing. It's a '78 MGB with a Mallory dual point distributer, a Weber carb and little or no smog control devices left. When the outside temp is down it runs fine. Drive all over the place with no problems. Driving down the road this afternoon (it's HOT around here) it was just like turning the ignition key off. Tach dropped to zero and everything died. Pulled off the road to check things out and noticed the map light on when I opened the door. Tried to satrt it and it fired up. Drove 100 feet and it died again. Fiddled with the key and it managed to start again. Drove the last 8 miles home and numerous times it stumbled and caught itself. Each time the tach dropped to zero. It seems like there's something under the bonnet that causes a power breakdown when the underbonnet temperature reaches a certain level. As soon as the sun goes down and things cool off I'm sure it'll run fine all night. Got me stumped.
Tom

Coils typically don't fail but it sure sounds like that could be it as they fail when they get hot. I assume external temp could effect it. I am sure there are possible other causes...see what other people say, check the cheapest to fix first. If you can get a good coil to put in temporarily is the best way to see. Testing of coils requires equipment beyond the usual multi-tester, I am told. Sounds frustrating and I am familiar with the experience of intermittent fault.
J.T. Bamford

The fact that it's happening in hot weather makes me wonder if there's a hydrolock in the fuel line? Had a similar thing happen to an old Toyota driving through Texas a few years ago.
Tony

You have to narrow your problem down. When the car stops, what other electrical things die at the same time? Everything - including the headlights? Just the switched things like wipers and radio? Just the motor and tachometer? Just the motore?
Matt Kulka

Tom. The main indicator you mention is the tach dropping to zero. I am assuming that you mean it drops immediately rather than winding down. This is normally an indication of an electrical problem in the low tension circuit. That would be from the power input to the ignition switch to the white wire circuit, including the ignition relay mounted in front of the fuse box. You have either a bad connection, a broken wire or the ignition relay or coil are breaking down. If you can duplicate the failures in the garage, you should be able to use a volt meter to trace where the power is and is not. Les
Les Bengtson

I had the a similar problem this year. Turned out to be no more than a loose, dirty connection at the coil.
Brad Batchelor

Tom, check your wireing under the dash. Had a similar problem on my 73B. Cruseing down the road and the engine acts like the key was turned off. Found a loose cannon plug coming out of the ignition switch. Cleaned it up and put it together...runs great.
doug
Doug Gordon

Further to what Les points out if the ignition warning light also came on as well as the tach dropping instantly that is telling you the supply through the ignition switch onto the white has failed. But if the warning light didn't come on, and it normally works as it should, then it is a break further towards the coil and through that and whatever type of distributor you have. If it is a points system it could be points or the distributor ground strap, if one of the factory electronic ignition systems then any of the wiring around those.
Paul Hunt

On the other hand...
I had similar problems with my '67. I'd be cruising along and then... silence...
I found that clicking the ignition key off and on again "popped" loose the sticking points on the fuel pump and I'd be fine for awhile again.
Of course I got to that point after several fruitless attempts at figuring out the problem in other systems.
Another source of trouble could be the ignition points, although the stock system in the '78 didn't have any, you now have a Mallory so you now have twice as many things to fail. I replaced the breaker plate with a YL (breakerless) and have been very happy (Obviously I also have the Mallory).

Mike!
mike!

This thread was discussed between 13/07/2004 and 19/07/2004

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