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MG MGB Technical - Run-on and Back-fire

Could someone enlighten me on the proper operation of the "anti-run-on valve", why does it fail/malfunction, the relationship between it and back-firing and how ambient temperature plays into these factors?
DK Kiernan

DK,

the anti run on valve is opened when the oil pressure droppes when ignition is switched off to stop the engine.
It ventilates the inlet manifold this way when operated. If the valve is striking in the closed position, run on is possible as it is if it is not connected to the oil presure switch or if the oil presure swith does not work (switches only when there is no oil presure present).
Some simple checks will help to locate the mistake in the system.
BTW also check the pipes that run from the valve to the manifold and the other one for blockage.

Ralph
Ralph

DK - Go to Paul Hunt's Pages of B and V http://www.mgb-stuff.org.uk/ click on Spanners, then on Fuel and scroll down to 'Run-on'. Paul has a very comprehensive write up on the problem.

Backfiring is caused by ar getting into the exhaust system, mixing with unburnd fuel sufficiently to produce an explosive mixture just waiting for a spark to ignite it. Cheers - Dave
David DuBois

"Backfiring is caused by ar getting into the exhaust system, mixing with unburnd fuel sufficiently to produce an explosive mixture just waiting for a spark to ignite it. Cheers - Dave " ---

Dave,
If I put a glow plug in the silencer do you think I might benifit from the "after burner" effect? It'd sure look neat to have flames coming out from the tail pipe! (lol)

glg
glg

DK. Run-on can also be associated with a too high warm idle speed. Should be under 1,000 rpm.

Backfiring, as Dave notes, is associated with fuel burning after it has been exhausted from the cylinders. Part of this can come about, as when shifting or decelerating, because the throttle plate is nearly closed, resulting in a higher vacuum pressure there, which allows droplets of fuel inside the intake manifold, to be drawn into the cylinders, producing an excessively rich mixture. The function of the gulp valve is to weaken the mixture under such conditions and prevent backfire.

Les
Les Bengtson

The MGB anti runon system is dependant on *all* the emissions plumbing and parts being attached and not blocked, as well as electrical connections from the ignition switch and an oil pressure switch - a normally open one, not the usual normally closed type used for an oil warning light.

The valve operates when the ignition is off and while there is oil pressure in the engine. It *closes* the port that allows air into the canister, and that allows vacuum from the inlet manifold to be applied to the float chamber(s) which sucks the fuel out of the jets, it is this which stops the engine running-on.

However there is a design error in the wiring of 77 and later cars which means this is the *only* way of turning off the engine, which means that any problem in the anti-runon system allows the engine to continue to run normally for a while after the ignition is switched off. There is a wiring change that can be done to overcome this (NOT the infamous diode in the ignition warning light frig) which was how the problem was solved on UK cars.

There is an after-market anti-runon valve which does what Ralph says i.e. opens and allows air into the inlet manifold, but I found it made no difference to the severe Dieselling I got when changing to unleaded fuel. Eventually I used it in a similar way to the North American system and that works fine.
Paul Hunt 2

I have a 1973 mgb w/45 dcoe side draft weber.I get alot of backfiring and popping when i let off the gas. The car has no emission equipment at all. any suggestions?
m charatan

have a 1973 mgb w/45 dcoe side draft weber.I get alot of backfiring and popping when i let off the gas. The car has no emission equipment at all. any suggestions
Well...
Does it have a fast road cam? If so this is unburnt fuel burning in the exhaust, just like those short stacked aero engines on finals in the war films. If so it is to be enjoyed!!
Stan Best

I'm not sure if it has a fast road cam. Does the car need any of the emission equipment to run correctly? ie: gulp valve, air pump or anything else?
m charatan

The car will run with none of the above. The Dodge Copperhead concept had it's FI programmed to bleed a little fuel on the overun, I guess they were going for the ageing hot rodder segment.
Stan Best

This thread was discussed between 22/06/2007 and 29/06/2007

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