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MG MGB Technical - Anti run-on valve

1974.5 MGB, PO installed Weber DGV carb conversion, most pollution equipment removed except for the carbon cannister and anti run-on valve and associated plumbing. During some tinkering I managed to break off the 2 contacts on top of the valve and have a used valve coming which I bought on eBay. I researched the archives and am assuming that since the valve hoses are not connected to the carbs, it was designed for the S.U.s, it is not doing anything, i.e. not sucking the fuel from the float bowl to prevent running on since it is not connected to the carb at all. This is how I understand how the anti run-on, carbon cannister, fuel system works. The carbon cannister absorbs the fumes from the carb while the car is not running, when running the anti run-on valve is open to allow fresh air to be drawn into the cannister and into the valve cover to dispose of the fumes. When car is turned off the anti run-on valve closes and in conjuction with the vacuum from the engine sucks the fuel from the carb float bowls, starving the engine, no gas, no run, engine off. Since my anti run-on valve is not connected to the carb it is then not needed and is basicly doing nothing, correct? And thus I wasted my money buying a used valve as it has no function if not set up properly. Also, the carbon cannister is only acting as a filter for valve cover ventilation, right? Does anyone know a way to connect to a Weber DGV to the carbon cannister plumbing, or should I not bother at all. Sorry for the long post, this is confusing to me.
J Mancone


The carbon canister absorbs vapor from the fuel tank, basically all the time but particularlly when the car is not running. When the car is started the anti-run-on valve opens the base of the canister to the atmosphere and the vapors are pulled into the carb either through the PCV or air cleaner, or both. When the car is turned off the run-on valve closes to the canister and opens to the manifold creating a mixture too lean to burn.

The Weber manifold has an extra hole that could be plumbed to the run-on valve, but most people don't use it because excessive run-on can be solved in other ways. The run-on valve is still good for helping to remove the gas vapor from the canister. If your canister is like the one on my '77 then it has three nipples on the top. The central one is plumbed to the fuel tank (though it doesn't matter which one really), originally one was connected to the valve cover, and the last went to the air intake. You can continue to go both ways or only plumb one way and the vapors will still be pulled into the carb and burned. If you have the rectangular K&N air filter there will be a hole in one corner of the base. The PCV may be routed into it. You could add a tee and a line to the canister top for a PCV/air-cleaner draw.

D. Cook

Thanks D, I have some things to sort out under the hood.
J. Mancone

For the actual way the anti-runon valve works go to the Pages of Bee and Vee at click on Spanners, then on Fuel, then on Running on. Cheers - Dave
David DuBois

Thanks for the response Dave but my question real is since the anti run-on valve is in no way connected to the carb then it is not performing its function, since its function is to suck the fuel out of the carb and into the carbon cannister causing the engine to stop and preventing run on. Upon start up the valve opens to let fresh air run through the cannister and back to the engine to be burned up lowering emmissions. So, since it is not doing this then it is not needed. Since I have a Weber DGV there is no way to connect it back to the carb (I could connect it to the nipple on the intake manifold but that still isn't what it is intended for) then it is only opening to let fresh air into the carbon cannister and into the valve cover. Am I any way near correct on this matter?
J Mancone

J, your understanding of how it works is pretty close to mine. If not connected to the carb float chamber vent then indeed it is doing nothing and it not required. The charcoal canister will be limiting the fumes that can get to the atmosphere through expansion of the fuel in the tank (but not the carb which it originally would have been), more than preventing engine fumes from getting to the atmosphere. If the rest of the plumbing is in place then all you should need is a pipe from the canister to the float chamber vent. The pipe from the anti-runon valve to close by the carbs is to a small drilling in the inlet manifold. With a manifold for a Weber carb you may not have that, which would need more work to provide, and if you aren't running-on it hardly seems worth it. The valve isn't needed to remove the vapours from the canister, that is done by the suction from the valve cover, which will still happen whether the valve is present or not. But to get *that* you need a suction pipe from the carb to the front tappet chest cover, do you have that with your Weber? If not you have no crankcase ventilation, which is a bad thing as it will allow condensation to build up.
Paul Hunt

Paul, I don't have the car with me at this time (I'm at work trying to look busy) but I do know I have a hose running from the front of the engine block to the carb air cleaner, and thus to the carb. This hose (I'm not sure what it's called) vents the engine block as oil fumes comes from it when the car is running. (I'm am not a mechanic so my descriptions may sound a little basic). I started this thread due to the fact that while messing around under the hood (I like to call it therapy) I broke both connector points on top of the valve. I tried to solder them but lets just say my soldering skills are lacking. I bought a used valve on eBay for $20 (after I saw the price of a new one it was quite a bargain) and now I realize I didn't need it at all. Oh well, $20 isn't a lot of money, and I'll think of it as a learning tool. Oh, by the way Paul, I use your web site constantly, it is saved in my favorites, and I did consult it prior to this posting. It is in my opinion one of the best sources for information on MGs and their workings and you should be proud of it.
J Mancone

If the hose only goes to the air cleaner then whilst you will get some crankcase ventilation it will be little more than burning any fumes that happen to make their way up the pipe to it, it won't really be scavenging the crankcase or canister. This was the system until early in 64 when the closed-circuit system was introduced with the PCV valve, the valve being replaced by the much better carb suction in late 68. If your carb doesn't have a suction port then I'd consider retro fitting a PCV valve. Thanks for the comments, J, I appreciate it.
Paul Hunt

This thread was discussed between 12/04/2005 and 14/04/2005

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