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MG MGB Technical - Correct fuel pressure to carbs

Have a 69 B all stock. Recently installed a new fuel pump and resealed carbs. At first start up gas was pouring from overflow tubes, checked fuel pressure and read 3 lbs out of the pump. Is this too mch pressure? Someone once told me years ago pressure to the carbs should only be 1 to 1 1/2 lbs. Is this information correct?

It should really not be any more than 2 1/2 psi.
Just got finished dealing with this issue.
MK Mike

The AZX 1307 SU fuel pump for MGBs puts out 2.7psi. there is another model (I don't know what the number is off the top of my head) that puts out 3.8psi. While the AZX 1307 is specified for the MGB, the other one with 3.8psi will work fine. Anything less than 4psi and greater than 1.5psi should work in a MGB. Cheers - Dave
David DuBois

I have an in line regulator, set to it's lowest value (not tonight, it's too cold to go and read it) it still fuels OK
Stan Best

I've seen electronic pumps advertised recently that boast much higher pressures than this. I'm wondering how much the float chamber needles will tolerate.

Brian McIlvenna

"I've seen electronic pumps advertised recently that boast much higher pressures than this" Those are not SU fuel pumps. As long as one stays with SU fuel pumps, you will not over pressurize the carburetor needle valves. After market pumps that put out more than 4 psi need to have a pressure regulator installed to reduce the pressure. Cheers - Dave
David DuBois

Well, now I'm also curious about my fuel pump pressure - as I have an after market electronic one(little brass looking square, mounted near heater motor) with SU carbs.

I wonder if the pressure from this pump is too high.

Although, my in-line see-through fuel filter never does fill up all the way, and actually runs quite low at times, so there can't be too much pressure, but I'm not sure.

Something I'll have to look into before I move the pump from near firewall where it pulls fuel from tank to back by battery where it can push fuel like it should be.

Just another thing to add to my SPRING check list.
R.W Anderson

RW - It doesn't matter how much pressure is applied, the filter will take many hours of constant running to get it to fill up (on my test stand it takes as much as 10 or 12 hours for the filter to fill completely. The little, square, run all the time, make a lot of noise, Facet pump you are running is rated between 2.5 - 4 psi, wiell withing the range that the SU carburetors will tolerate.
Cheers - Dave
David DuBois

My filter varies between almost completely empty to almost completely full when observed periodically. Ignition on or off, engine running or not, makes very little difference to the fuel level in the filter except for a slight pulse as the pump clicks. Fuel level in the filter seems to have no bearing on engine performance, and pump pressure will have almost no bearing on fuel level in the filter.

If the pressure is to high it will overwhelm the float valves and flood the engine and/or pour out of the vent tubes/charcoal canister. Other than that pump pressure also has no effect on engine performance, it is flow-rate which is important. The standard SU should deliver up to 2 Imperial pints per minute at the carbs (a minimum of 1 Imp pt) which is way more than enough for anything likely to be seen in an MGB on the public roads.
Paul Hunt 2

I re-read this today, then went and looked at the regulator. It was set to 2.5 psi, however since it is quite close to the carbs there is no need for all that pressure as the fuel has already been pumped past the pressure drop in the fuel line. I have set it down to 2 psi , and may try it at 1.5 in the summer.
Stan Best

I've got a Facet Red-Top (6-7psi) with the regulator 'set' to maximum which is 6psi. There is no overflow.

Here's an update guys.... My gauge starts reading at 4psi, output pressure at the pump barely made the needle move which indicates to me its 2 to 3 psi with pressure regulator removed. BTW the carbs are AUD326 with needle AAE. The rear carb runs richer than the front. Took both dash pots off and with the ignition on and can see fuel slightly bubble from the jets, more so at the rear carb. The front jet has a green band and the rear has no band both are new from Moss. The needles have no wear marks or scratches. Both pistons move free in the dashpots and make the audiable thump when they hit the bridge. One thing I did notice is there is a mark on the collar that hold the needle in the piston and both marks were aligned in different positions. Repositioned them tonight so the needle bias is towards the rear of the piston and the alignment mark is set between the holes on the back side of the piston however, have not started the car to see how it runs. Would this cause an extreme rich mixture? Is the fuel pressure too high? I have seen new su pumps in the past with unusually high pressure, atleast for this pump.

Okay so here is an update. The pump pressure is 5.5 psi. Not sure what model it is. Hate to install a regulator but have no choice.

This thread was discussed between 12/02/2008 and 22/02/2008

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