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MG MGB Technical - engine runs on

I have finally got my 1976 mgb running again the reason it would not run was the rotor arm must have been the wrong one. A trip to my local carparts shop sold me the correct one and it is several millimetres longer and when fitted it started first time. however it seems that now, when I switch off, the engine runs on for a second or two longer than before. Any ideas why this should be. I have been tidying up the wiring also as there were a few nicks in the insulation here and there.thanks in advance for your comments Trev
trev walton

Check for vacuum leaks in the anti-runon system.
Kimberly

Kimberly, a '76 UK car probably doesn't have an anti run-on system. We still had twin SUs right to the end of production and very little of the anti-smog gear you were forced to take in the US.

Trev, what fuel are you using? Are you using any additive? Running-on is a constant theme because of modern fuels. The B was designed for minimum 99 octane leaded fuel. You MIGHT prevent it by retarding the ignition a degree or two, but I simply used to jam my right foot on the brake, my left on the clutch, select second gear, and as I turned off gently stall the engine with the clutch. Much kinder to the mechanicals. Eventually it went away when I bought and fitted a Burgess cylinder head which was modified to cope with modern fuel.
Mike Howlett

I have seen this when the timing is too far off. I would check that first. Temporarily, an alternative method to what Mike was saying, giving the throttle a quick blip right before cutting the car off will help prevent it. (not a permanent fix though)
Bryan

With all due respect, how can the timing make a difference when the ignition is off?
Tom

Tom, the timing will affect cylinder temperature when running. The run on is caused by a cylinder temerature hot enough to ignight the fuel mixture without a spark from the plug. Advance the timing a a little at a time and see if it reduces the run on. If you get any pinking you will need to back off a little on the advance.
John H

Trev,

From what I gathered when I asked the same question a few months ago, this is not an uncommon problem. In addition to what has been said above, getting your idle speed as low as possible will help. If that and proper tuning does not stop it, then the higher octane fuel may be the cure.

If you want to read plenty more, it's in the archives.

Charley
C R Huff

My 73 got so bad after changing to unleaded, even on the higher octane, I had to fit an anti-runon valve. An idle speed slightly higher than spec made it worse, but it was bad at even the correct idle. At lower than that it shuddered and shook, and when stuck in traffic would tend to stall. Blipping the throttle, or flooring it as I switched off, made no difference and I hated doing that anyway. Stalling it in 4th (safer than 2nd) was the only clean way, but I hated doing that as well. After fitting the valve (it's an after-market, but plumbed in as instructed did nothing, I had to invent my own system based on the US valve) I can now run a slightly higher idle than normal to avoid stalling, and get a clean switch-off.
Paul Hunt 2

"the timing will affect cylinder temperature when running. The run on is caused by a cylinder temerature hot enough to ignight the fuel mixture without a spark from the plug. Advance the timing a a little at a time and see if it reduces the run on. If you get any pinking you will need to back off a little on the advance."

The other affect that timing plays in run on is that a retarded spark will lower the idle speed. IN order to compensate for the lower idle speed, it is necessary to crank the idle screw further down, which then leaves the throttle plate open a bit more at shutdown. Advancing the timing as far as possible and then decreasing the idle speed with the idle screw is often all that is needed to eliminate (or at least minimize) the run on problem. Cheers - Dave
David DuBois

Retarded ignition also causes hotter combustion chambers and hence more chance of run-on from glowing bits in the combustion chamber. These can be lumps of carbon or shards of metal. The basic design of the combustion chamber includes a wedge between the valve seats. The tip of this can glow red hot, and there are recommendations to grind this down slightly, *reducing* compression ratio, but solving problems with run-on and pinking, the latter allowing you to run with a bit more advance which compensates for the loss of compression.
Paul Hunt 2

This thread was discussed between 21/04/2008 and 24/04/2008

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