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MG MGB Technical - '73 ignition switch wires

Hello All, My '73 ignition tumbler S... the Bed.
I have 2 other complete steering columns,'71-72'or so
but neither have the slate wire which is hot at only key off.From what I understand this is for anti-run on..valve? on a '73? Can I delete this wire? Will the other colapsable columns be interchangeable, or just the tumbler, & switch?
Len Fanelli

Yes, I'm running my 74 without the slate wire to the anti-run on valve. Look in front of the evaporative cannister for a cylinder about 1 inch diameter and 3 inches tall with hoses connected to the evaporative cannister. There will be 2 wires connected to the top of the anti-run on valve. It's mounted on a 1 by 1 inch metal shelf that sticks out over the edge of the bulkhead.
Unplug the wires and drive the car a bit to see if there's any problem with running on, or dieseling. If not, then the car will run okay without it.
When you shut the key off, the switch in the anti-run on valve shuts off the air flow to the engine, which causes the engine to shut off immediately because the crankcase can't breathe. As long as the car doesn't run-on, then you're fine without the valve.
I used an earlier switch which didn't have the slate wire (and cost about half as much), and I've been driving it for about 2 years without the anti run on valve and it does just fine.
Good luck!
1974 mgb roadster
1973 MGB parts car
1990 Miata
Kenneth Thompson

The anti-runon valve was provided for the 1973 model year, engine types 18V672 and 673. However it wasn't really vital until the 77 model year when the ignition relay was provided, which included as an undocumented feature the fact that *only* the anti-runon valve would stop the engine when the ignition was turned off, not the ignition itself.

It works not by shutting off the air supply, but by shutting off the *fuel* supply. It causes a small vacuum to be applied to the float chamber vents, which sucks the fuel out of the jets, hence preventing any chance of run-on. It is so effective that the wiring problem mentioned above only came to light when any part of the emissions control system had a problem. That and UK cars with emissiosn control where the problem was discovered immediatelyk, corrected with a wiring mod, which was never applied to US cars.

If you don't get running-on without it then fine. If you do then you can reproduce the function of the missing slate wire with a relay with a normally closed contact powered from the ignition white, the normally closed contact connecting the brown to the slate when the ignition is off and the relay released.

My 73 UK car developed such bad running-on when we changed to unleaded that I had to use the USA methodology to stop it.
Paul Hunt

This thread was discussed between 30/08/2009 and 11/09/2009

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