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MG MGB Technical - Anti Run On Valve

Converting to twin SU's, on an 80 roadster, do I still need the Anti Run On Valve, or can I dump it.

If still needed, what should it now be hooked up to, as all the emmission stuff is off.



Jack

Hi Jack.

If it doesn't run on after a drive that gets it good and hot on a warm day, you don't need it.

It works by leaking air into the inlet manifold (which is where you need to connect it), making the mixture too weak to ignite.

HTH... Don
Don

Jack-
Don is thinking of the Gulp Valve. The Antirun-on Valve doesn't "leak air into the intake manifold" as the Gulp Valve does. Vacuum produced by the induction system is applied to the adsorption cannister which in turn is hooked up to the Antirun-on Valve. When the ignition is shut off, the Antirun-on Valve opens and vacuum in the adsorption cannister is applied to the chambers of the carburetor float bowls. This vacuum is greater than that produced in the carburetors, so fuel is not drawn into the carburetors and the engine stalls.
Steve S.

I may have dumped the adsorption cannister.

Will now frantically search for it.

Steve, you say vacuum produced by the induction system is applied to the adsorption cannister, which in turn is hooked up to the Antirun-on Valve.

Can you please explain what I need to retain of the emmission stuff, bearing in mind I want rid of as much as possible.

What is the induction system.
Jack

Steve,
On the later MGB's the anit-runon valve is connected to the absorption cannisters, the large diameter plastic fresh air tube running to the front of the car through the radiator support and the intake manifold. It was also my impression that fresh air through the large tube entered the intake manifold thereby diluting the mix. How exactly do the float chambers see the vacuum you are talking about? Is it the direction is reversed...the vacuum at the intake manifold travels through the valve..then the absorption cannister and finally its connection to the carb?
Frank

Looks like I mispoke myself. I quote from page 412 of the Bentley manual: "When the ignition is switched off the solenoid is energized through an oil pressure switch ond the valve closes, shutting off the ventilation connection" (to the adsorption cannister) "and opening the connection to the inlet manifold. Inlet manifold depression then acts on the fuel in the carburetter float chamber(s) to prevent fuel flow and the engine is stopped by fuel starvation." So, the vacuum is supplied directly from the induction system, not the adsorption cannister.
Steve S.

I removed everything on my 1978 roadster. Air pump, canisters, all tubes and pipes. The only thing I left was the tappet cover outlet to the carb connection. I havent noticed anything unusual and the car runs well now. The idle may be a bit low. I replaced the stock carb with a weber adn put stainless tube headers on. So far so good.
CW Strong

The canister has no effect on the engine performance. If it and the associated pipes to the fuel vapor separator are removed you will be venting fuel vapors into the trunk or under the car depenting on where the pipe removal stops. Or I guess you could install a vented fuel cap. FWIW, Clifton
Clifton Gordon

Jack,
I suggest you hang onto your anti run on as it can be useful. Re plumb it a bit though. It should be connected to the intake manifold. when switched off (ignition off)it essentially opens the intake manifold and decreases the vacuum, the SU pistons will drop down and close off the jets, leaning the mix to nothing. When switched on it will close off the "vacuum leak".
Why I say "keep it" is that prior to putting in the V8 I had a "SU converted pollution gear ripped off ex American low comp 1800 motor" and in the hotter months it definitly used to run on a turn or two. I played around with the timeing and thermostat until it more or less was fixed, but an anti run on valve would have been nice.
Also Clifton is quite right, you do need some sort of vent to or from your fuel tank as the American caps seal the filler neck. Otherwise pressure will either build up in there in hot days, or as the tank empties quite a vacuum will form, pobably wouldn't crush the tank as the SU pumps not that good, but might cause fuel starvation.
Conecting the purge line and the other "intake" end of the anti run on valve to the top of the canister uses it simply as an air filter but there's no reason why the two lines need to be co-conected other than expediency (you need some sort of air filter for each so why not). There is no need to put the canister under vacuum (another line to the manifold, via crankcase) unless you need to meet emission regulations.
Peter

Good idea about the fuel tank, I never thought about pressure there, I have a loosely fitting gas cap now..that is probably why I dont have any problem. I kept everything that came off the engine, its in a box to pass on to the next new owner. If what I read is correct, then I should put the charcoal canister back in the car and put the fuel vent back in it. Is that correct?

CW Strong

The evaporative loss system which includes the air tight filler cap, charcoal canister, and run-on valve does not take power from the engine to operate. Moss catalogues have a good illustration of the system.

The run-on valve came along in 1973, and operates with oil pressure switch. The canister is easily re-built with new activated charcoal, screen, and gauze filter. If in good working order keep it in place for cleaner air for all of us.

Regards,

L.C. '74 B/GT
Larry C.

CW-
It is important to retain the crankcase ventilation system. Properly maintained, crankcase gases are drawn into the combustion chambers of the engine by the vacuum created by the fuel induction system, either through the intake manifold as in the 18GB-18GF engines, or through the carburetors as in the later engines. This permits the crankcase to function in a partial vacuum which causes oil mist inside the crankcase to be drawn upwards towards the camshaft and tappets. Without the vacuum provided by this system, the pressurized gases inside the crankcase of the B Series engine would cause oil to be blown past the pistons into the combustion chambers leading to carbon buildup and consequent preignition problems. In addition, an excess of these pressurized gases and oil mist would also be vented partially through its rocker arm cover, decreasing the vacuum in the carbon cannister and interfering with its function, rather than traveling down through the pushrod passages as they should to aid in the lubrication of the lower ball ends of the pushrods and the upper sections of the tappets. For the excess pressurized gases in the crankcase to arrive at the rocker arm cover they would have to travel up the past the pushrods. This means that the gases would be forced upward around the tappets, depriving their upper sections of the additional lubrication supplied by the oil mist and the oil running down the pushrods from the rocker arm assembly. It thus must be understood that all of this is prevented by drawing all of the pressurized gases inside the engine out through the tappet chest cover and into the induction system under vacuum, and as such the system contributes to prolonging engine lifespan. The front tappet chest cover from the late 18V engines is preferable due to its better breathing characteristics and the oil reservoir/return chamber incorporated into the cover design.
Steve S.

Boy is this confusing, I need another drink to clear my head.

It seems that the consensus is that this Anti run-on Valve is a good thing, however if I retain it, can someone just indicate what I need to connect it to, in the engine compartment, sounds like the SU carbs, anything else.?

I will not be reconnecting the plumbing to the tank, it went in the trash ages ago ( mechanics fault I hasten to add).
Jack

Here is a link with a schematic of the fuel system.

Perhaps it might be helpful to indicate which of these pipes etc remain on an unfettered set-up using twin SU's.

http://www.w-p-c.com/vapor.htm
Tom

Jack-
Hook up a hose from the small tube on the side of the valve body to the intake manifold to supply the vacuum. Hook up another hose on the opposite side of the valve and run it to the carburetor float bowls. Attach a small filter to the restrictor tube on the rocker arm cover to perform the filering previously done by the charcoal canister.
Steve S.

Several things:

When the ignition is turned off it is the *ignition switch* that operates the valve, the oil pressure switch *releases* it after the engine has stopped and oil pressure has died away.

The valve *closes* a fresh air inlet when it operates. This allows suction from the carbs and inlet manifold to be applied to the carb float chambers, sucking the fuel out of the jets, and instantly stopping the engine.

During normal running the valve is open, allowing fresh air to be pulled through the canister, clearing the products of expansion vapours from the tank and float chambers, then through the rocker cover and crankcase pulling any oil vapours from there, all to be burnt in the cylinders. There are restrictions in the system to prevent the fresh air drawn in this way from diluting the mixture too much.

When the ignition relay was provided there was a wiring fault such that turning off the ignition did not cut the supply of sparks to the plugs. Normally this would have allowed the engine to continue to run normally (not Dieselling, although it prevents that too) for a while, but the anti-runon system is so effective at sucking the fuel out of the jets no one knew about the wiring fault - until a problem develops in the anti-runon valve or emissions plumbing means you can't stop the engine with the key.

Paul Hunt

Have now found in my pile of old parts an electric device, square with a couple of power spade male conectors.

This device has a long, say 12" very thin tube attached to it, plastic I think, and looks as though it might be long enough to reach over to the carb position.

It definitely is part of the anti run on valve.

Question is what is it and do I retain it notwithstanding the above comments.
Jack

This thread was discussed between 01/09/2003 and 09/09/2003

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