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MG MGB Technical - Fuel issue

Hi Folks,
I have a 1972 B GT which on firing up leaks petrol from one of the carb float chamber pipes that runs for approx 12inches from the top of the float chamber. Anyone any ideas on reason and a fix ?

Thx a lot.
J McCaughey

You most likely have some small debris that is holding the metering needle open , in the float chamber, that is leaking fuel. Which carbs do you have, the HS4s with the side mounted float chamber or the HIF4s with the internal floats? The HS4s are easier to service, as the float lid comes off with the removal of 3 screws. RAY
rjm RAY

Thx Ray,
HS4s with the side mounted float chamber, I'll take a look over the weekend :)
J McCaughey

I fixed my overflow problems with Viton tipped needles. This has been the subject of several livley debates among members, all in the archives :-)
Stan Best

I have been having a lot of problems with my HIF6s on my V8 overflowing and I have replaced the needle's and seats a couple of times. I currently have the Viton tipped needles.
Out of frustration yesterday I put a pressure guage on the electronic SU fuel pump I have and it shows I have 4 1/2 psi. I always thought SUs pumps put out 2 or 3 psi.
I cannot vouch for the exact accuracy of the guage but I do believe it is accurate and has a scale of 0 to 10psi.
I rang the Australian SU distributor and he said the pressure was a little high but that should not be a problem and suggested another needle and seat.
It is always the 'second' carburettor that overflows for some reason.
Do you folks think the pressure is too high?
Ian Buckley

Ian - The output of a AZX 1300 series pump is either 2.7psi or 3.8psi. Your pump may be one of the 3.8 psi models. It is also rather difficult to measure the output pressure of a SU pump since it is an impulse type of pump, which can read quit high on the peak of the impulse. Regardless, the float chamber should shut off even at 4.5psi. Have you checked that there is no fuel in the float on the carburetor in question. Cheers - Dave
David DuBois

David, thank you and yes I have checked the floats myself and the carburettors have also been looked at by the local SU specialist. The Australian distributor of SU said that normally the output can be up to 3.8psi but this can be lowered by fitting a different spring.
I am seeking a permanent fix and I am tempted to send the pump to them to have the pressure lowered.


As I type this I suddenly had a flashback and remembered I had another low pressure guage in the garage. I just went out there and disconnected the pipe from the carburettors and hooked it to this other guage which is new and 'Made in Australia since 1935'. So I will assume it is more accurate than the earlier guage. This new guage has a scale of 1 - 15psi and the pressure shown is 2.7.

So now I am nowhere in solving my problem of the overflowing needles and seats. What I failed to mention earlier is that the needles and seats I have been fitting are Fuel Miser brand with the Viton tips because my local SU man sells them because he has had so much trouble with original SU ones.
Ian Buckley

Disconnect the fuel pump where the white wire from the main harness joins the rear harness by the fusebox. Run the engine until it empties the carbs and stops. Reconnect the fuel pump and the resulting rush of fuel through the now wide open float valves may clear any debris. If it happens again straight away remove the float chamber lid and check the float isn't punctured and containing fuel. If that is OK then replace the float valve. If it's OK for a while and then starts flooding again it could be dirty fuel from a rusted tank or pipes (although SU pumps contain filters), which will mean muck in the bottom of the float chambers. As a quick fix you could insert a fuel filter in the engine bay (UK cars didn't get one as standard until later).
PaulH Solihull

Ian - If the pump has a tag with the model number affixed to it, check the number. If it is a AZX 1307, then the output pressure is 2.7psi - if it is AZX 1308, the output pressure is 3.8psi. Cheers - Dave
David DuBois

Thanks folks I will try Paul's technique and also see if I can get one of these guages tested to see what fuel pressure I really have. Plus as David says I will check the pump tag.
Ian Buckley

I had the second guage I used to test the fuel pump pressure tested and it showed 7psi at a real 10 psi, so my Australian made guage is not as accurate as I hoped.
I also just removed the pump because it was too hard to see the tag and it is AZX 1307 which is supposed to be 2.7 psi. I think I will still send it back to SU to get it checked over and perhaps a spring fitted which will give me the correct pressure. There is a slight weeping around the diaphram cover so getting it checked will be worthwhile.
Ian Buckley

I'd be very surprised if the AZX1307 delivered 10psi instead of 2.7, unless someone had modified it. A heavier return spring would indeed deliver more pressure, but then it could well need a heftier solenoid to pull the diaphragm back against that spring pressure. Fuel pressure is developed from the return spring of the diaphragm, and nothing else. However pressure will rise if there is heat build-up in the pipe between pump and carb once the pump is applying pressure, if the carbs aren't using any.
PaulH Solihull

This thread was discussed between 10/06/2011 and 17/06/2011

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