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MG MGB Technical - Fuel Pressure at HIF Carbs

Hi Guys, Some time ago there was a a thread re the pressure delivered from SU pump AT the HIF carbs. As I recall the optimum was 2.5PSI.
Can someone please confirm this, thanks.
I have not found the thread in the archives for 2009/10.
I have an issue with the replacement pump which is running at 6 to 7PSI and flooding both carbs.
Is the HIF float too small (lacking flotation capacity) to control the higher pressure? The needle and seats have been replaced with no change.

Thanks for the assisatnce.
Best regards
Ray Bester
R. E Bester

Ray - 6 - 7 psi is way too much pressure. The original SU fuel pumps only produced 2.7 psi. You need a pump that provides that amount up to no more than 4.0 psi (a good candidate would be an original SU pump). Cheers - Dave
David DuBois

As an aside can a pressre regulator cure this or do they just cure minor fluctuations in pressure? The filter king that was mentioned on other threads cam to mind? Mike
J.M. Doust

It really is so much simpler to stick with an SU fuel pump, especially one of the newer electronic ones. They deliver sufficient fuel at the right pressure and will even supply ample fuel for a V8 running SU HIFs.

I recently returned my SU pump to the distributors for checking and it is pumping at 3.7 psi and the HIFs seem to be happy with that.
Ian Buckley

Mike - I have never tried to fuss with a regulator, but from reports that I read on the various forums, they are not real reliable. Better to get a pump that puts out the correct pressure.

Ian - there are 2 variations of the AZX 1300 series pumps - a AZX 1307 with an output pressure of 2.7 psi (which is the pump shown for the MGB in the Burlen Fuel Systems catalog) and a AZX 1308 with an output pressure of 3.8psi and I believe is used on some of the Jags. Both versions work fine on MGBs from my experience. Cheers - Dave
David DuBois

My vote is with the correct pump as well, adding a regulator is adding time, money, and potential problems. Even pointy pumps will run for very many years without problem, and the points are easy to deal with. I have had my old roadster pump on the V8 for many years without any issues, when the pointless pump started playing up.
PaulH Solihull

David, it turns out mine is the AZX 1307 which as you say, should be 2.7 psi. I was having overflow problems for a long time and in my frustration I returned my pump to be checked and it was pumping 3.7 psi. My problem was the floats were upside down. I discussed that at the time in a thread on the V8 heading. My floats were put back in the wrong carburettors by the local professional.

Interestingly the car ran ok most of the time.

Ian
Ian Buckley

Ian - According to the Burlen Fuel Systems reference, the AZX 1307 has an output pressure of 2.7psi and the AZX 1308 has an output pressure of 3.8psi. That said, I have received AZX 1307 pumps that have the 3.8psi volute spring. I have always chalked this up to someone having overhauled the pump and put the wrong volute spring into it (very easy to do unless on measures the diameter of the spring wire). Considering what you have written, I have to wonder if perhaps the springs could be getting mixed up at the plant from time to time. Personally, I don't see a reason for having two pressure specs that are so close together as it doesn't seem to make a difference to the carburetors.

Things do get a bit more confusing when dealing with the all electronic pumps. The negative ground pumps are as follows: AZX1307EN - 2.7psi, AZX1318EN - 3.8psi. Positive ground pump: AZX1308EP - 2.7psi, AZX1319 - 3.8psi. This is enough to make a fellow dizzy. Cheers - Dave
David DuBois

Hi Guys thanks for the advise. I will take the issue up with HUCO Pumps who supplied the "wrong/ too high" pressure pump as a direct replacement for the SU. They have a 2.2 PSI in the range so I dont see that they can have too many objections.


Thanks again

Best regards
Ray Bester

PS I hope some of you make it to the MG Indaba in South Africa in a little town called Clarens in the Free state in September 2012.



R. E Bester

David - not saying you are wrong, just making an observation that the pressures you quote are illogical. With the points pumps 1307 2.7 and 1308 being 3.8, and the 1307EN being 2.7 one would expect the 1308EP to be 3.8 and not 2.7.
PaulH Solihull

Paul - That is the reason for my comment "This is enough to make a fellow dizzy." I took these specifications directly off the Burlen Fuel Systems web site. The problem of course is that there are two designations for each of the all electronic pumps - I would have handled that differently, but they didn't consult me :-) It might be a good idea to check the BFS web site again as there could well have been a misprint when I got the information initially (a couple of years ago). Cheers - Dave
David DuBois

My vote is for the original SU pump.
Only had to replace the original one after 37years of use.

Fitted a new one and the GT worked fine again.
Oh and it also became a tad more economical?!
Onno Könemann

This thread was discussed between 26/09/2011 and 02/10/2011

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.