Here's an explanation I saw on an MGB bulletin board a while back:
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From: Paul Kile, Fair Oaks, CA on 29/1/1997 14:36
George, here's the full scoop on the anti-run on valve. The valve doesn't
just open a passage into the intake manifold, it pulls manifold vacuum on
the carburetter float chamber(s). This prevents fuel from entering the jets
and stalls the engine. The way this occurs is somewhat complex:
1. Engine running condition - the valve is de-energised and is open between
the carbon canister connection and the big hose at the bottom of the valve.
This allows the canister to vent to atmosphere. Since the canister connects
to the carb float bowl vent(s), the float bowls are vented to atmosphere.
The engine runs.
2. Key is switched off - power is supplied to the valve through the
dedicated oil pressure sensing switch (which completes the circuit to
ground), which simultaneously closes off the big bottom hose connection and
opens the manifold vacuum connection. This applies manifold vacuum instead
of atmospheric air to the carbon canister, causing a lower pressure above
the float bowl(s) and preventing any more fuel from reaching the carb
venturi(s). The engine stalls.
3. Oil pressure drops after engine stalls - This opens the oil pressure
sensing switch, cutting the power through the valve, and resetting it to
the "engine running" condition. That's the click you hear a few seconds
after the engine quits.
It took us years to figure this out....
Cheers, PK
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Other possible causes listed on the board: low octane fuel, carbon
build-up, high tickover, a weak mixture (air leak or vacuum advance unit
diaphragm
split) or out of adjustment points/timing.
--Steve
At 01:29 PM 11/1/98 EST, you wrote:
>Hi, all. I hope someone can help me with this.
>
>Recently my car has developed a problem with running-on. What might be
>causing this? Is there a way to check out the anti-run on valve. Actually,
>what the hell does that valve do?
>
>I would greatly appreciate any help.
>
>Cheers,
>
>Jeremy
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