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MG MGB Technical - run on

hi, i want to fit an anti run on valve to my mgc. is it ok to take live from back of ignition switch.
regards bob.
bob taylor

Easier from the fusebox, the ignition-switched circuit.
paulh4

Bob, I have a friend who had a TR4 which ran on wildly. The air dump valve didn't cure it. We threw the kitchen sink at it, i.e., retarded the ignition by a couple of degrees, tweaked the carbs a touch richer. put colder plugs in and he started using 97ron fuel. Something worked!!
Allan Reeling

The valve purchased from the MGOC did nothing for my roadster either, installed as recommended in the servo vacuum hose with a Tee, so I came up with my own system - based on the American valve - that sucks the fuel out of the carb jets on switch-off and has totally cured the problem.

Subsequently I discovered that with the valve attached to the inlet manifold using a PCV port in a spare hole that also worked.

Mine had always tended to runon with 4-star leaded, but got much worse on unleaded, even 97-99 octane. So much worse that I had to do something about it as the 'recommended' alternative of flooring the throttle at switch off was hit and miss and didn't appeal to me, stalling it in 4th before switch off even less.
paulh4

hi allan,
valve hasn't cured my problem either. done the bits you suggested except colder plugs, do you think its worth a try, got 6bpes at the moment.
thanks bob.
bob taylor

Bob, run on or pre-ignition, sometimes called dieseling, is usually a result of a hot spot in the combustion chamber carrying on igniting mixture until it cools. The very end of the thin section of the combustion chamber between the valves can glow, (not sure whether C's have the "kidney" design) as will carbon build up, casting irregularities and the plug tips. Colder plugs run slightly cooler because of better heat conduction to the cylinder head and might help. The down side is if your engine is an oil burner, the plugs might tend to foul. With NGK the higher the number, the cooler the plug. It's worth always bearing in mind that the fuel does lend a hand in controlling combustion chamber temperatures. Keeping the float chambers insulated from the exhaust will help too.
Allan Reeling

Lower octane fuels here in times past meant run-on was something we became very familiar with. The suggestions above are certainly valid, but the most common cause by far is air leaking through worn throttle spindles. This affects two ways: by introducing a varying amount of air which leans it off when the engine is turned off, and by needing to increase engine speed significantly to get a consistent idle. I have covered various other causes at http://www.mgparts.co.nz/advice/technical-notes/run-on/

Regarding ignition timing, I have found that advance is better than retard. Retarding the spark slows the engine, requiring more throttle to maintain an idle, so delivering more fuel so self ignite when the engine is turned off.

Advance has the opposite effect, and is most noticeable when working with the late cars with manifold vacuum take-of - disconnect the vacuum advance and the idle speed drops away. So I have been able to solve some stubborn cases in early cars by shifting the vacuum from carb to manifold, allowing the throttle opening to be reduced for a given idles speed.
Paul Walbran

Paul,
Understand your point about the throttle spindles, but thought that would be less of a problem with HIF's due to the seals (assuming they are there of course and not too hard). Doesn't over advancing increase combustion chamber pressure, hence temperature? What may appear to be the WM timing is often way off because of the crank pulley inner and outer have shifted as the rubber deteriorates. Inertia shifts the outer anti-clockwise, i.e., advancing. If retarding a couple of degrees helps the run-on then the next step is to check the TDC as indicated on the pulley.
Allan Reeling

The seals are fairly token and are certainly better than nothing, but I have seen plenty of HIFs where new spindles and bushes have cured run-on.

Yes, over-advancing does increase temperature, so also does retard. Best conditions are at optimum amount of advance. Cars with ported vacuum run relatively retarded at idle so will accept more idle advance. This can't be done by advancing the whole distributor as then the engine would be over-advanced under load, hence a solution of shifting the vacuum take-off to the manifold. As a comparison, early chrome bumper Bs so modified have similar idle advance to later rubber bumper cars.

Very good point about the crank pulley!
Paul Walbran

I had an MGB with a real bad run-on. I finally cured it by removing the air filters and using a small funnel with the engine running I poured ATF into the carbs till the engine ran real rough. I then shut it off and let it rest overnight.
Started it up the next day and had a 20 foot long black streak up my driveway. This cured it completely. It had soaked and softened the carbon build-up and blew it out on my driveway.
I learned then to use an old potato sack to cover the exhaust pipe and collect all the gunk.
Sandy
Sandy Saunders

Paul is spot on, retarding ignition makes em run on worse. Low idle and the correct idle advance is needed as well as not too weak. However, if you fit an anti run on valve the idle mix needs to be as weak as possible or the engine will still run on with the richer mix and extra oxygen!
Peter
Peter Burgess Tuning

There is one solution I have yet to see fail to cure run on - simply floor the accelerator pedal as you turn off.
Chris at Octarine Services

This thread was discussed between 13/10/2016 and 15/11/2016

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