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MG MGB Technical - Antirun-on valve

How does the antirun-on valve function to prevent run-on and damage to the engine.
Dick Field

Dick,
never had one myself but I understand it opens when the ignition is switched off, thereby allowing air in to the manifold. This causes a very lean mix of any unused fuel and prevents combustion and hence run on.

Best of....
MGmike

M McAndrew

Going by memory here, but as I recall, if it detects oil pressure whilst the ignition is off (i.e. the ignition is off but the engine is still spinning, therefore it has just been turned off) it applies vacuum to the carb bowl vent which sucks the fuel down & out of the jet. No fuel, no run-on.
Rob Edwards

have this from Paul Hunt's excellent site mgb-stuff - http://www.mgb-stuff.org.uk/fueltext.htm#runonna2
Nigel Atkins

Rob's correct, it doesn't dump air into the manifold but sucks the fuel out of the jets. It is so effective that for years it wasn't discovered that there is a wiring design error on North American spec cars with the ignition relay which means that turning off the ignition key doesn't kill the sparks to the plugs until the engine has stopped spinning.
Paul Hunt

All,

Thanks, this helps a lot. I'm converting my 1979 to twin HS4s. With this specific information, I'll hook the bowl vents on each unit to the top of one canister and vac from the manifold to the antrun-on valve itself.

Dick
Dick Field

Depending on how you wire it up, this can also be used as an anti theft devise with some secret switching incorporated in the circuit. I fitted one from a Ford Escort once into a Triumph Spitfire, which worked very well. I must confess that I thought it worked by letting the atmospheric pressure into the inlet manifold. Barrie E
B Egerton

When converting from single Stromberg to twin SU all you should need to do is put a splitter in the float chamber vent tube so it can connect to both carbs. Do the same with the crankcase breather hose.

By earthing the oil pressure switch wire with a secret switch you operate the anti-runon valve as soon as you turn on the ignition, and that should prevent it starting. But all the emissions plumbing has to be present and correct or you may not get enough vacuum to do the job.

Another neat trick on 73 and later cars without the anti-runon circuit is to earth the signal wire at the tach with the secret switch, which kills the sparks.
Paul Hunt

This thread was discussed between 01/12/2013 and 03/12/2013

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