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MG MGB Technical - petrol pump problem

Help please....
1972 MGB HIF carbs, nearly ready for the road. Trying to tune the carbs, hadn't even got the engine warm, when i noticed that fuel was pouring past the jet on the rear carb, into the cylinder, yes it stopped running and i will have to replace the oil. I currently reak of petrol after draining most of the tank and taking the pump off. Before i did this i disconnected the pipe at the petrol filter and couldn't stop the flow with my finger over the end. So, not the carbs but the pump. the pump is in bits, but when the wires are connected the points work ok and the diaphram seems to go alright. Everything else has been cleaned. What should i look for. This can't happen when i'm on the road ! Should i fit a aftermarket electric pump? Dam, tuning the carbs was the last thing before the sticker and now this.
Please help...
Gerry

Gerry
I can't see how the tank or pump would cause this but a quick check with a fuel pressure gauge would tell if things are normal there. If all is ok then it will either be that your carburettor float has a hole in it and sunk allowing the needle to come off it's seat and flood the carb. or it could be a faulty needle and seat(or incorrect float level) or some foreign object (rubbish) has worked it's way through the fuel lines to the needle and seat and stuck there holding the needle off it's seat It would be very rare for a standard fuel pump to pump over pressure.
Hope this helps Gary
Gary

Gerry - The next time you have an occasion to remove a pump with a full tank of fuel, disconnect (or just loosen the fitting of) the fuel line at the upper right side of the tank, that will break the siphon on the tank and save you from a bath in fuel.

Gary states that "It would be very rare for a standard fuel pump to pump over pressure". I will go further, it is impossible for a SU fuel pump to put out more than the rated pressure (in the case of the AUF 300/AZX 1300 series pumps used in the later MGBs, the pressure is 2.7 psi). Since the pump is an impulse type of pump, it is very difficult to get a definitive pressure reading on it, but the pressure is determined by the volute spring behind the diaphragm and if anything, the spring gets weaker over time (in over 30 years of working on SU fuel pumps, I have never seen one put out too much pressure). Gerry has listed the most probably cause of your fuel leak in the carburetor, I would only add that the pivot screw for the float can work loose and cause the float to go out of position and not shut off the fuel. Cheers - Dave
David DuBois

Gerry. If only the rear carb is over flowing, it indicates a bad needle valve in the rear carb. This is assuming that you have an SU fuel pump on the car--you do not mention what pump you have. With the standard SU pump, there is no problem with too much pressure. With some of the after market pumps there may be.

Before David DuBois organized his own website, he had some fuel system tech articles on my website, www.custompistols.com/car. These have been replaced by a link to his website, through which you can contact Dave. Send him an e-mail and tell him what fuel pump you have and what problems you are having. He has, always, been good about providing assistance.

Les
Les Bengtson

A lot in the archives about this. I used teflon (viton) tiped needles for a while then changed to a pressure reducing valve in the fuel pipe. It seems to be modern fuel formulations as there was nothing the standard valves could do to stop it.
Stan Best

Thanks for the input guys. You all seem to think its the carb, which i'll check later. But on the pump side. I thought that, partly becuase i'd have thought by putting your finger over the end of the petrol pipe from the filter in the engine bay, it would have made the pump think the carbs didn't need any more fuel and stopped pumping. It jetted past my finger with somew pressure.
Dave thanks for the advise with the fitting to stop the flow from the tank, the house still smells of petrol this morning.
Gerry

As David says it is impossible for an *SU* pump to put out excessive pressure as the only pressure comes from a return spring. If that were too heavy I doubt the solenoid would be able to pull it back. I suggest petrol continued to leak past your thumb due to a combination of roughness of thumb or end of the hose/pipe, or an unseen split close by. Or you already have an aftermarket pump of some kind, these *are* known to overwhelm the float valves and need a pressure reducing valve to be installed before the carbs.

Normally when the float valve is faulty petrol pours out of the vent/overflow. If this was not happening I suggest you may have the supply and vent pipes reversed, or a blocked vent pipe. Certainly in the latter case this prevents the float rising to shut fuel off, and fuel is pumped straight up out of the jet.
Paul Hunt

Gerry - Les and Paul brought up two items that I didn't think about. When you talked about installing an after market pump, I assumed that you have a SU fuel pump installed. that could be a bad assumption, because you didn't specifically state that the pump is a SU pump. As Les states, you may already have an after market pump installed that has too much pressure. Paul points out, correctly, that the overflowing fuel should exit through the overflow tube, either on the ground or into the charcoal canister. A plugged overflow tube could in fact cause the over flowing of fuel into the carburetor throat even though the needle valve is working correctly. In addition to providing a path for excess fuel to overflow from the float bowl, it also vents the float bowl and if it is clogged, fuel will be pushed from the float bowl right into the carburetor throat even if it (float bowl) is not full enough to shut off the flow from the pump. Cheers - Dave
David DuBois

Gerry.
By any chance do you have GROSE JETS? The old ones were fantastic but the newer ones are nothing but crap. I would use the viton tipped needles and make certain that you do not exceed 2.7 lbs pressure. I had a seventy two model that even with 1 lb pressure the grose jets would leak. I chased that problem with three sets until I replaced float valves with Viton tips. No further problems.
Sandy
Sandy Sanders

Gerry,
You have the principle of fuel flow regulation wrong. The fuel pump will pump fuel all the time the ignition is on. That is the ticking noise you hear when you turn on your car. It ticks faster at first, as it fills up the float bowls. It slows down when the floats rise and force the needles to seat into the fuel entry jet to the bowl. The float rises, the needle blocks the flow of fuel and the carb uses the fuel in the bowl. As fuel moves into the engine, the float lowers and the needle opens the fuel feed a little to let more fuel into the bowl. You will notice that the pump continues to tick if you turn the ignition on and the bowls fill up because it needs to maintain pressure in the lines for when the fuel in the bowls is used and the needle opens to let more fuel in. By disconnecting the fuel line near the carbs, and trying to fully block the line, you are trying to stop the 2.7 lbs/sq.in. pressure that should always be there. Thats why it tried to spray past your finger! Maybe this will help you understand what was happening.
David Burke

Thanks everybody. Yes the pump is original. I'll take the carbs off tonight and give them a complete clean, and follow all of the above
Gerry

Gerry
Don't overlook Paul's suggestion that the fuel supply and vent hoses could be mixed up. It's happened before and the result is a lot like what you've got there.
Gary
Gary

David (Burke) - that is not correct, the pump will only pump fuel *as it is required*. When the ignition is on but the engine stopped the float chambers fill, the float valves close, and the resultant back-pressure in the lines stops the pump pumping any more. In this state the pump may not click for several minutes, or at the very most not more than once every 30 secs. Even when the engine is running the pump will only click occasionally as it delivers fuel in pulses and one pulse will keep the float chambers full and the fuel line pressurised for several seconds - depending on throttle opening and revs. That is for SU pumps which Gerry has. Certain aftermarket pumps *do* click or run continuously, but not SUs, but even then they still don't pump fuel unless it is requited by the float chambers. The above also isn't strictly fuel regulation but fuel delivery. Regulation is different, and is required as an extra valve for some (most?) after-market pumps (but not SU) to prevent flooding.

The only pumps I'm aware of that *do* pump fuel all the time are FI pumps which have a return pipe back to the tank from the injection system.
Paul Hunt

This thread was discussed between 01/06/2008 and 03/06/2008

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