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MG MGB Technical - Dumping Petrol

Can someone please explain how the petrol feed to the SUs works? Currently with the ignition on but engine not started (ie fuel pump active) the pump simply fills up the float chambers and then dumps petrol to the ground via the metal pipes on the other side of the float inlet.
I have recently renewed the float chamber valve.
Kenneth Herbert Ralph Baron

Hmmm. When things are working right it functions just like your toilet tank: there's a float, and when the fuel reaches a certain level the float rises and a valve shuts off to prevent any more fuel from coming in. Simple enough. So why would the valve fail to close?

Normally one might assume a faulty float or valve, but since you just replaced the valve, and you surely would have noticed a punctured float on doing this, I suspect the air is not venting out - if the float bowl vent is plugged, there's no place for air to go as the fuel comes in. Air pressure will build up in the float bowl and force fuel out any available orifice - fuel will come out all kinds of places it shouldn't. I think that's probably what's happening here. So check to see if you might have capped off something you shouldn't have. (on Cal-spec vehicles the float bowl vent line went to the charcoal canister, but depending on the year it might go to different places)

Kenneth. The float level is not set correctly or your new valves are bad. If the overflow pipes were plugged, you would not be getting fuel flowing from them. Rather, it would be forced out through the jets.

There are several things to look at. First, what fuel pump are you running? The SU pumps, with pressure and flow specifications are detailed in a tech article by David DuBois on my website. Go to then the MG section and click on "articles". If you have a non-standard fuel pump, or one putting out excessive pressure, the floats cannot cause the needle valve to seal even when working properly.

If the fuel pump pressure is within specifications, it has to be the floats, the valve or the height adjustment. Check each of them to find out where the problem is. Les
Les Bengtson

Wow - that was quick. Thanks Sam and Les, you've given me some useful leads. Of course I should have said that it is a 1973 MGB GT with twin HS4 (I think) SUs. So nothing as esoteric as charcoal canisters at that time. But is the SU float chamber vented? Or does this lead-off pipe act as the vent?
Yes I'll check the tolerance on the float level - I recall seeing that in the manual, and it looks as though I can alter it with washers under the brass barrel of the valve. I did test the action of the valve, by blowing through the inlet and it SEEMED to shut off when the float was raised. But you are right Les, I must check the precise height adjustment.

Actually, I have recently bought a Solid-State switched SU pump to replace the normal points one that is installed - but the latter pumped petrol through at a healthy rate (on to the floor!) - so I am fairly sure that the pump itself is not the problem, as it must be about 5 years old and I thought it was at about replacement time. So I would be surprised if it was developing excessive pressure. I have replaced the fuel line with a new 4 piece SU braided steel covering the pipeline. The bits seemed a fraction long, and this may have pushed the float chambers into an angle as they pivot at the tube connecting them to the carb. Would this create a problem?
Thanks again for your help. I'll have another go.
Ken Baron

Ken - The lead off pipes, as you call them, are the vents for the float bowl. I doubt that they are clogged, since if they were you would 1) not see fuel overflowing through them and 2) the excess fuel would be coming out of the jet in the affected carburetor. I suspect that Les is correct when he says that the needle valve may be set too high and the means to correct that is with extra washers (the reason there are several supplied with the valve). You stated that you recently replaced the needle valve, was that because the float bowl was overflowing or was there another reason that you replaced it? Do check the float itself to be sure that it is not full of petrol, as that would definitely keep it from doing it's job. The other thing is that the pivot pin could have come dropped out, unseen as you turned the lid over into position to install it. If this happens, it could do one of two things. 1) If the pin fell completely out, the float would not be pivoted and will not shut off the valve. 2) If the pin only cam part way out, it could jam the float so that it won't rise or it could have been caught on the edge of the float bowl, such that the lid is not seated correctly (not likely as that would allow the petrol to come out the edge of the lid. Good luck - Dave
PS. You stated that the original fuel pump is 5 years old so you could be one trip away from it failing or it may very well serve you well for another 15 years (or, of course anything in between)
David DuBois

UK cars never had the charcoal canister, which is only something in the overflow/vent path anyway. As David has said if fuel is coming out of the pipes they are not blocked, so it is a float or valve problem. Whilst in the normal scheme of things 5 years is nothing to a pump any pump can fail at any time, and in my experience a pointless has been more unreliable than a standard one. But fuel pouring out of the overflow(s) is *not* a pump problem if it is a standard pump or a pukka electronic replacement.

Sometime electrically disconnecting the pump and running the engine until the float bowls have emptied, then reconnecting the pump with the now wide-open float valves, can dislodge any momentary bit of debris. But if it happens again immediately either the float or valve is at fault, and if it seems to do the trick but happens again shortly then maybe you have a contaminated tank and dirty fuel. Fitting a fuel filter (UK 73s at least didn't have one from the factory) can resolve this, but very dirty fuel i.e. from a badly rusted tank could block the filter fairly quickly and cause fuel starvation.
Paul Hunt

I had to fit neoprene tipped float valves to stop my car doing this. New steel ones had no effect. Floats were fine and a new pump did not help either .
S Best

Well, this information exchange certainly works! I cannot thank you all enough for helping me with this problem. I have resolved it!
I replaced both floatchamber valves, (the guy at Moss said that there were no adjustment washers for ensuring the 1/8 in clearance described in the manual) - so I just went ahead and replaced them with no adjustment (actually on a visual look at the clearance it was about 1/8" anyway). Cost about 8.5 pounds each which was excessive - but I just did it. Now that is problem is completely solved.
Thanks to everyone who contributed.
Ken Baron

This thread was discussed between 13/01/2005 and 22/01/2005

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