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MG MGA - Problem with new SU Fuel Pump

I changed both carburetors and fuel pump and replaced them with brand new original SU spares.
I am now experiencing the same problems of fuel flooding as described here :

http://www.chicagolandmgclub.com/techtips/general/558.html

I did also install the grosejets but i think this is typically the wrong solution as the problem is that the brand new SU fuel pump developping too much fuel pressure.

I wonder if anyone did experience the same faulty parts and is there anything worth to do ?



Marcel

Marcel
I had the same problem with the fuel pump on my 74B. The regulator solved the problem.
Kris Sorensen

Marcel,
This is not likely a faulty pump at all, but the same situation I described in the thread "Help How do I fix this?" from Robert Browning on the MGB tech board, 27 August.
The pump only develops pressure by compressing a spring - it is the spring that controls the pressure. There would have to be an incorrect spring fitted, which is very unlikely.
A regulator as suggested in the link you gave may or may not correct the problem. One of the 4 cars that lead me to figure this out had a regulator, and it didn't help at all. The problem is that if the fuel is boiling, you will have vapor or liquid at different points in the system. The regulator doesn't seem to deal with vapor very well, and if the boiling is happening downstream from the regulator, the regulator has no effect. Even if the regulator manages to maintain the correct pressure with the float bowls full, the lines may only have vapor in them, which means that as soon as you start off and the floats drop, there is only vapor to fill them. Now you have fuel starvation until some liquid arrives. When it does, the float valves close, the fuel boils, the pressure goes up, the regulator tries to cut pressure, the carbs flood - a mess! The problem is mostly at low speed or idle, since there is very little fuel flow or air flow along the lines to keep things cool.
The only things you can do to help this are: 1) be sure there are no missing heat shields, 2)no exhaust leaks or modifications that may heat fuel lines 3)Wait until it's cooler, or 4)Make a bypass system as I described.
FRM
FR Millmore

Thanks for you views Kris and FrM, I do not wish to add a regulator unless it is the only solution I get.

Just want to add I have no problem of heat shield : fuel flooding arises even when starting a cold engine .

I checked the levels of both floats .OK.
It was flooding from the rear carb at first. Added a grose jet and now it is flooding from the front carb.

I also shall add a second grose jet there but the above thread seems to indicate it will be useless.

It is not normal that brand new SU remanufactured pump have a pressure 3 or 4 times above the original specs. Something is wrong (may be that incorrect spring you mentioned FrM ).A faulty part for Barney Gaylord's collection ...

Marcel

Have you tested the pressure developed by the fuel pump? Provided you have purchased the correct pump it is unlikely that a new pump would be developing too much pressure.
Keith Morris

Marcel - I would have to say that it is totally impossible for a SU fuel pump to produce a pressure 3 to 4 times the normal pressure due to the design ofthe pump. The pressure is determined by the volute spring and is either 2.7 psi or 3.8 psi, depending on the spring used. I do not know of any SU fuel pump that puts out a pressure any greater than 3.8 psi. To get a pressure that is up to 4 times greater than the specified pressure would require a volute spring that would be so strong that the solenoid coil would have trouble pulling the diaphragm in. Have you checked your floats themselves to see if they have developed leaks and may be partially full of fuel? Good luck - Dave
David DuBois

Marcel,
Now you are saying "brand new SU remanufactured pump". Remanufactured by whom? If it was SU, then I agree with Dave. If by someone else, then the only way for this to happen is that the "remanufacturer" put in a different spring, or maybe stretched the correct one (a Lot!) The only other possibility is that the tank vent is blocked and there is pressure in the tank.
FRM
FR Millmore

Hello Keith and David,

Both the carbs and the fuel pumps are brand new and manufactured ans sold as new by SU UK.

Therefore the floats are also new , so were the needle ans seats I swapped for grosejets.

The chicagoland link I mentionned states:
"I hurried home, dug out the pressure tester and my worries were over. 12 1/2 lbs. of pressure were coming out of old faithful."

That 12 1/2 PSI is raughly 3 times more than the 2.7 PSI expected...
I have no tools to mesure the exact pressure but it looks like being the same problem.

FRM, I will check about the tank vent you mentionned. I don't know where this vent is supposed to be. Can you say more ?

Thanks
Marcel
Marcel

Hi Marcel

Leaking needle valves are a very common problem (especially in a newly assembled setup where some dirt, rust etc may have got into the tank or pipes) and something you should, in my opinion, eliminate as a cause of your problem before worrying about the fuel pump.

Cheers
Keith
Keith Morris

Marcel,
I believe the MGA is vented through the cap - a small hole in the center baffle you can see, and another to vent from there to the outside edge of the gasket. It has been a while since I looked at an A cap, and you may have some non-stock replacement. Leaving the cap open is a good temporary way to eliminate this source of problems.
Note that the author of the link you give was in the same situation you are - he was describing a situation that is not possible - an SU pump putting out 12.5 psi. If it was happening, then everything we have told you applies. As I said in the thread I referenced, the only way this can happen is under hot running conditions, and it is not a pump fault.
You should measure the pressure at the carbs before condemning the fuel pump. You need a pressure gauge you can attach at the carb. The original braided lines are difficult to work with, but if you have an old one, you can carefully cut the ferrules off the ends and use rubber fuel hose to make a connection to a gauge. This can be used to check the pressure sitting still or while driving. You may be wasting your time trying to fix something which is not the problem. If the pressure is right, then you have a problem with float valve, float, or float level. Otherwise, your trouble is due to dirt fouling the float valves.
FRM
FR Millmore

This thread was discussed between 01/09/2005 and 03/09/2005

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