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MG MGA - anti run-on valve

A combination of suspect/low octane fuel(we are told it is 95) and a higher compression cyl. head I am experiencing pre-ignitioning when switching the ignition off on my 3brg engine MGB engine fitted to my MGA Supersport has anyone had success with an anti run-on valve plumbed into the inlet manifold?


Not me, but I too will be interested to listen to the comments. However, I have all but eliminated run-on in my later 1800 with super unleaded (98+ octane). Engine thrives on it, no hint of pinking and I am told the engine will run slightly cooler. But it is expensive at 0.98 ($1.90) a litre over here.

When I was forced to use 95 a while ago, I used to reduce the risk of run-on by letting the engine stabilise at idle for 10-15 seconds before switching off. I have also been known to drag the clutch in first at switch-off, but no doubt someone will say that is not good practice!

Steve Gyles

Anyone here in the states ever think that Oh Boy, gas is only $2.00 a gallon. Our current price here in California is about $3.35 a gallon. In this state we can not import gas from other states as our gas is made to state regulation for less smog.

Trevor. Paul Hunt's website, The Pages of Bee and Vee, has a tech article on installing an anti-run on valve. Too high an idle speed will tend to cause run on which might not be present at a lower idle speed. Might be worth checking both out.

Les Bengtson

I seem to remember an anti-run on valve being part of the later MGB 1800 emission controls. Perhaps thats a place to start.
G Goeppner

Jeff- Steve still has us beat, a litre I think is about equal to our quart,so he's paying about 7.60 a gallon
gary starr

I have not had experience with an anti run-on valve on an MGA, but but have had them for many years on Triumph six cylinder engines. They have always worked well.
You need the valve and a special relay. When you turn the engine off the relay activates the valve, which then applies engine manifold vacuum to the overflow pipes of the SU's. This raises the fuel level in the SU bowls by pulling fuel away from the SU jets, stopping the engine (or put another way, it prevents the airflow over the jets from picking up fuel).
I think that the Triumph six cylinder saloons were sold in South Africa. Is that correct?
If so, there should be the correct fittings in car dismantling yards. The MGB had the same system, but used parts would be harder to find.

Mick Anderson

In my previous post I only mentioned the vacuum part of the anti run-on system. The electrical part is a bit more complex and may make it difficult to fit to a MGA.
The relay is a type where the internal points open a circuit when the power is applied and close a circuit when the power is removed.
It works as follows:
When ignition power is applied to the relay, as when the engine is running, the relay points are open and no power is passed to the anti run-on valve.
When you turn the ignition off, the relay points close and pass power from the non-ignition part of the loom to the relay.
The anti run-on valve operates and applies manifold vacuum to the SU overflow pipes.
Problem! With the ignition off you would always have current passing through the valve coil, even when the car is left for long periods.
Solution. On the Triumph with the anti run-on valve there is an extra terminal on the oil pressure sensor switch on the engine block. This switch can now also act as an earth for the valve coil. As soon as the engine stops turning the oil pressure drops to zero, the earth is broken, and the valve coil no longer has current flow.
When testing a Triumph anti run-on valve, you put your finger in the opening at the bottom of the valve and push the spring loaded plunger. The engine stops instantly.
Simple method for a MGA. Use only the valve, no special relay nor special oil pressure sensor.
Have power from a non-ignition part of the circuit to a pushbutton type switch. This can pass power to the anti run-on valve.
Press the button to stop the engine. Release when engine is stationary and then turn off the ignition.
You need to do something, run-on will damage the engine.

Mick Anderson

I also use the clutch drag method. With my car run-on is only a problem with a hot engine and I find that it takes only a slight drag on the cluth when turning the ignition off to bring things to a smooth halt.
Ed Bell

Keep the idle speed low, not more than 800rpm to avoid run on. Not keen on stalling the engine to stop it as this can unnecessarily wind up the transmission and cause premature wear on the clutch damper springs and universal joints. It will also stress the transmission and rear axle and if you have wire wheels could aggravate hub spline wear. It's better to avoid that method
Iain MacKintosh

I do not stall the engine with the clutch, only put a slight drag on it to slow the engine when turning the ignition off. Less drag than a normal start from standing still.
Ed Bell

This thread was discussed between 11/04/2007 and 13/04/2007

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