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MG MGB Technical - Air in fuel regulator

I recently replaced the fuel pump on my 1972 MGB GT with an official MG catologue product. It created coughing and spluttering after a couple of weeks and started to see fuel coming out of the float reservoir overflow pipes. I had to have the car trailed home bcs it just refused to fire. I came to the conclusion it was pumping too much petrol into the carbs so I bought the MG catalogue Filter King regulator and fitted that. I bought new gaskets for under the float lids and cleaned the reserviors out inside and checked the float movement. Everything appears to be in order. I topped up the SU carbs with oil, cleaned the K&N filters and gave the car a standard service. The car started and I took it for a spin. It went great for about 2 miles then had the same problem. I managed to get home at 3mph!Under the bonnet there are no leaks but I saw bubbles coming from the bottom of the glass regulator and floating to the top at intervals of about 3/4 seconds. The regulator is 7/8ths full with petrol. Question: are the bubbles the problem for the spluttering and stalling? If so where could they be coming from? Is the fuel level in the regulator still too much?
Any help would be appreciated.
Kevin Fleetwood

you shouldn't need a fuel regulator at all

did you check the carb needles and seats for wear or sticking

I'd check the flow rate of the fuel pump (just before the new fitted fuel regulator) how much clear uninterrupted flow in 15 or 30 seconds

is the petrol cap vented, if so is it working, easy to check, remove it and run the engine

I'm also thinking of muck in supply line, or perhaps clips that need nipping up or a very small hole for air getting in rather than fuel leaking out

I wouldn't worry too much about how much petrol is in the regulator filter as long as it enter and exists without problem

did you set up the regulator as per instructions
Nigel Atkins

Everything Nigel says plus one. The engine runs on the fuel in the float chambers. The pump simply has to keep them topped up, so if the odd bubble gets through the engine won't even notice. If the float chanbers are overflowing it could well be the needle valve above the float. If that doesn't seal the chamber will definitely overflow.
Mike Howlett

And two. The club shouldn't sell such things for cars that don't need it, by advertising them they are spreading Fear, Uncertainty and Doubt, a classic (no pun intended) marketing policy.

It's possible changing the pump disturbed some sediment that lodged in the float valves, and caused the overflow from the float chambers. On the other hand it could simply be coincidence. Then again, if the pump is to the wrong spec it could well be supplying too much pressure and overwhelming the float valves, after-market pumps frequently do this.

I can't speak for the filter king but certainly the in-line filters on HIF-equipped cars can contain air from time to time, from seemingly containing all air and no fuel, to containing all fuel and no air, and the engine will run perfectly normally. As far as I know the filter king regulates pressure, the fuel level is irrelevant.

You started by saying the carbs were overflowing, then after fitting the filter king when you had a running problem there were no leaks 'under the bonnet'. If you mean that this time the carbs weren't overflowing, then it sounds like a different problem, i.e. fuel starvation. That could be from a faulty pump, or a faulty filter king. You need to do a delivery check by removing a pipe from a carb (if the ignition has been on recently there will probably be a spurt of fuel), direct it into a container, and turn on the ignition. You should get a minimum of one Imperial pint per minute, and in practice at least double that, in a steady series of pulses with minimal bubbles. If that doesn't happen bypass the filter king and try again. If that fixes it the filter king is faulty. If it doesn't the pump is faulty.

Paul Hunt

Thanks guys for your comments. If I understand correctly it is the carb needles which create the right mixture between air and petrol, so logically if one or both of the needles are sticking open or closed, it will cause starving or floading? I will check them. How will I know if they are worn? There will be a bit too much play in the seating? I will re-check all the pipes and check the flow as suggested. In fitting the fuel regulator I was careful to follow the 'brief' instructions. The instructions talked about regulating the flow if not exactly right, by adjusting the pressure of the diaphram by turning the screw in the lid either clockwise, to increase the pressure or anti-clockwise to decrease the pressure.This should adjust the flooding or starvation problem. What isn't clear is : increasing the pressure cures the flooding by limiting the fuel flow and at the same time increases the airflow and viceversa?

Kevin Fleetwood

Once again I am not a regular contributor as I no longer own an MG, but when I did I fell for the "I need a regulator" trick after having huge problems with flooding. It fixed all my problems after about 3 weeks when the regulator stopped supplying fuel completely.

Threw it out and sorted out the carbies properly and had no further problems. The SU fitted to the MGB was never going to flood the carbs due to excess pressure, or the factory would never have fitted that type of pump.

Tony Oliver

Kevin, I think you are confused about needles and valves. You are right that the mixture in the SU carb is controlled by a tapered needle rising and falling, the amount it moves being governed by the air flow through the carb venturi. Those needles could not cause the fuel to overflow. They are attached to the large piston inside the carb and they don't get stuck. They could be out of adjustment, or the wrong type, but that would make the engine run poorly.

The needle valves are in the lid of each float chamber. When the float rises it presses on the valve and closes off the fuel supply until the float sinks again. The pump keeps up a constant low pressure against the needle valve so that as the float sinks and rises with fuel being drawn out by the engine, the needle valve opens and shuts, maintaining the correct level of fuel in the carb.

If the needle valve is stuck open, or not sealing when closed, then too much fuel will enter the float chamber and the car will have a rich mixture and the fuel can overflow down the overflow pipes. In the same way, if the floats are not set to the right level they could either shut off the needle valve too soon, or not soon enough. Either condition needs attention.

You could do a lot worse than to read through Paul Hunt's piece on the SU. Find it here
Mike Howlett

Are you using the regular SU pump? If so dump the regulator -it's one less thing to worry about (and isn't necessary as others have said)
Michael Beswick

what's a standard service, is anything near the service requirements listed and scheduled in the Driver's Handbook, Ref: 0052 here -

to clarify further needles and seats requirements I hope this John Twist vid helps - Needle and seat -
Nigel Atkins

ETA: sorry at the moment I can't find the vid where he compares different needles and seats to each others but it's in there nearby I think
Nigel Atkins

You may have been sent a high pressure SU fuel pump by mistake. They're only used in a very few applications. Check the number on the pump and make certain that you were sent the correct unit by matching the numbers in the catalog. RAY
rjm RAY

"You may have been sent a high pressure SU fuel pump by mistake."
The highest pressure that any SU fuel pump puts out is 3.8 PSI and that is designated as the pump for MGBs. If you purchased an after market pump, all bets are off.
David DuBois

What was the "official MG catalogue"?? What make was the pump?
As has been said, if "the product" is an SU pump, ditch the filter king and instal a simple "in-Line' fuel filter. That will sort most of the future crud in the needle valve problems, as a cause of flooding. I say"most" because muck not only comes from the tank, but can come from the internal break down of rubber fuel hoses.
Also setting up a filter king without an "in-line' pressure gauge is a very hit and miss affair, mostly miss!!. Besides crud in the needle valve and over-pressure feed, flooding can be caused by worn valves or porous float, i.e., fuel in float causing it to sit lower or even sink completely. The state of the needle valve and float are always the first things to check if flooding occurs.
Allan Reeling

David, I thought that SU manufactured a higher pressure fuel pump, in the range of 5 psi, for certain applications. Obviously, I was in error. RAY
rjm RAY

This thread was discussed between 01/05/2014 and 05/05/2014

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