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MG MGB Technical - Stuttering under acceleration, tacho goes nuts

Hello,

I've just bought a 1970 B GT, which generally seems fine (if possibly in need of a little tuning). It runs fine - I've done a good 100+ mile run with no problems. However, the other night when out for a run, I came across a strange problem. Under acceleration (usually when pulling away from lights or roundabout) the car kind of stutters, with the rev counter going nuts (fluctuating) and the ignition light flickering. Eventually, the car will pick up and the problem disappears. I should mention that the car is fitted with a Lumenition electronic ignition device.

Any ideas or suggestions? I must say that I'm a bit of an electrical novice, so feel free to point out anything obvious.

Thanks in advance!

Alastair
Alastair Clarke

Sounds like some thing breaking down in the ignition creating extra pulses which is affecting you tach. Check all the connections first.If you have access to the parts you may want to put the points and condenser back in and see if it goes away.
John H

I would start by checking the wires inside the distributor to see if one is worn and grounding against the body of the distributor. If the wire were to intermittently touch ground, you would lose ignition (engine stutters) and the tach would cut in and out. When the tach cuts in and out the needle can bounce around quite wildly.

Hope that helps.

Erick
Erick Vesterback

Or it could be an intermittent earth, this does sound like ignition LT wiring.
Stan Best

Check the brown lead wire down under
at the starter solenoid.

It's held on by a push-on tab connector
and it often wriggles loose.

When it does come loose - it does all
that you described (jumpy tach, flickering
lights, sputtering ignition...)
Daniel Wong

If the tach is flickering it is definitely ignition LT. If the ignition warning light (assuming it works normally, that is) is also flickering then it will be the 12v supply from the brown through the ignition switch onto the white that is the problem. But if only the tach (and engine) is affected then it is in the coil/distributor circuit. The distributor has a ground wire internally between the points plate and the body, and this and the coil wire going into the distributor body are continually flexed back and fore as you use the throttle, and whilst the ground wire can visibly fail with the points wire the conductors can break inside the insulation which isn't visible.

With a 72 and earlier the ignition feed goes via the tach pickup, so any intermittent connections here will have the same effect. If you temporarily jumper the white at the fusebox to the coil +ve (this will disable the tach altogether) and that stops the misfiring then the problem is more than likely at the tach. If it makes no difference then it is more than likely inside the distributor, and as you say it happens when you pull away from the lights i.e. you are using the throttle then this would be my guess.
Paul Hunt 2

Just a quick update - spent all day working on the car yesterday with a couple of mates. We cleaned all the electrical connections on the LT circuit, and the problem seems to have gone away! Additionally, we put some new plugs in, flushed the cooling system, replaced the thermostat (which was found to be non-working, and changed the engine oil.

It's getting there slowly! Mind you, it's never ending - the rear offside suspension squeaks now - time for some oil methinks!

Thanks for all your suggestions, which were really helpful.

Alastair
Alastair Clarke

I had a squeaking rear spring on the V8 when the roads were dry. I painted the leaf springs with Waxoyl and used a heat gun to get it to melt and soak penetrate the leaves and interleaving. After a few dozen miles of driving and further distribution it stopped and never came back. Ive just changed the springs after breaking one, and this time I treated them before fitting while I could lay them down on their sides which aids the distribution.
Paul Hunt 2

This thread was discussed between 20/09/2007 and 23/09/2007

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