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MG MGB Technical - fitting anti-run on valve

Hi

I just bought an after market anti-run on valve from ebay. Mine's a UK car and so never had one as standard. It's the same one as is available from the MGOC site
(http://www.mgocaccessories.co.uk/acatalog/MGOC_Accessories__Tuning_37.html if you're interested).

I don't have any instructions and other than knowing its a valve that opens/closes with ignition key I've got no idea how it works or where it fits. Does anyone have any pointers to a good website?

Cheers
Darren

D Lewis

Hi Darren.

The idea is that it allows air into the inlet manifold when the ignition is turned off.
The extra air reults in a very weak mixture which (theoretically) won't allow the engine to 'diesel' on, though I understand that it doesn't always provide a complete cure.
I would guess that part of the reason for this (in twin carb cars) is that the manifold balance pipe is too restrictive.

If your car has a brake servo the logical way of connecting the valve would be to 'tee' it into the servo vacuum pipe.

I believe that a significant factor in 'running on' is that modern high-detergent oils tend to coke up engines of older design which tend to burn more oil than modern ones.

Don
Don

The Brake vaccum line might not be best as this (on my old 1800) has a very fine opening into the manifold.
As Don said, the moment the ignition turns off, the valve opens wide and dumps a whole lot of fresh air into the intake manifold behind the carbies and leans it out to nothing. As big an opening as possible would be best. The valve seems pretty large.
There should be a variety of blanking bolts in the manifold to choose from and there are a large variety of brass hose fitting availiable (autoshop, hydrolics shop (best bet), hardware shop etc. Remember to place some kind of air filter on the valve. On the American cars they plumbed it into the charcol filter (see your haines manual). If you haven't that set up availiable then some sort of carby bypass to the main filter. Or maybe a sump breather?
Just a thought, I don't have my haines book with me and it's been a while since I switched to the fuel injection, but the SU's operate by manifold vacuum sucking up the little pistons inside them which pulls the needvalve open. Idealy you would locate the vacuum passage that opens into the SU piston chamber and plumb your valve there. Since there are two of them you'd need a T peice.
Peter

Darren - For an accurate description of how the original anti-run on valve works and is hooked up, go to Paul Hunt's web page, "the Pages of Bee and Vee at: http://www.mgb-stuff.org.uk/ click on Spanners, then Fuel and finally Running on. Good luck - Dave
David DuBois

The reference that Dave kindly mentions also includes my experiences with the MGOC valve - which when plumbed into the brake servo vacuum hose as recommended did absolutely nothing to prevent runon. Eventually I plumbed it in a similar way to the North American valve, which did take a bit of engineering, and has proved totally successful. Since then someone has suggested that if screwed directly onto the inlet manifold it may work better than when in the servo hose.
Paul Hunt 2

This thread was discussed between 13/12/2005 and 15/12/2005

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