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MG MGA - S.U. Fuel Pump

I thought I'd give my pump a present and treat it to a rebuild kit as the contact breaker assembly didn't have much life left. I bought the kit (EPK605) from Burlen but noticed that the new diaphragm construction was a bit different to the one I was replacing. Also the new diaphragm spring had fewer turns but was noticeably stronger. Anyway I continued refurbishing the pump, following the manual instructions to the letter and found that on connecting to a battery the pump stopped 'pumping' when the six coil housing screws were fully tightened up. When they were each backed off a few turns it started 'pumping' again.
Many hours and much reading later I've come to the conclusion that a previous owner has replaced the pump with a low pressure 'L' type instead of the proper high pressure 'HP' type. I assume that the new HP stronger spring is to create the higher pressure and because of the higher loading the diaphragm has to be somewhat heavier duty. However, I can't see that there are any other differences between a low and a high pressure pump. Certainly the coil housing appears to be the same for both and so do the contact breaker mechanism and sandwich plate. Therefore surely the pump should work with the new HP spring and diaphragm? I'm beginning to think that maybe I've been supplied with an incorrect diaphragm (there appear to be four different types) as it does seem that when correctly set the diaphragm protrudes from the coil housing too far and is perhaps fouling against the sandwich plate when screwed up.

I'm surprised that the 'L' type apparently worked in the car before because the max. delivery head is supposed to be only 6" compared with the 11 or 12" actual head.

Has anyone any ideas before I accept defeat and call on Burlen for help? Thanks Mike
m.j. moore

Mike,
Just curious, did you follow the diaphram stretching instuctions for a new diaphram? Are you sure you have the correct diaphram for your pump? I believe the main difference relates to the length of the screw in the diaphram. Also, I'm not sure how a heavier spring would improve pressure. Have you tried your old diaphram and spring to see if the problem goes away? Those diaphrams rarely fail. Mine is original even though I "rebuilt" the pump. Flow and pressure are at the high end of spec. You also might try mixing and matching the springs and diaphrams as a test. Also, make sure you have adequate current as well as voltage when you test the pump. I bench tested my pump using a battery charger with a 2 amp setting and it wouldn't run. Bumped it up to the 6 amp setting and it worked fine.
I certainly don't view calling the supplier for help as admitting defeat. That's part of what you're paying for.
Good luck,
GTF
G T Foster

Mike,

Such an experienced supplier as Burlen is unlikely to supply incorrect parts.
The question is, did you correctly identify to them the correct type of pump that you have?
As GT Foster said, did you carry the correct stretching procedure for the diaphragm? When you say that it operates when the screws are loose, but stops when you tighten the screws, it certainly appears the the diaphragm is not being fully extended and held there while the screws are tightened. It sounds as if you are letting the diaphragm relax and then tightening the screws.
Do you have an SU Manual describing the diaphragm fitting procedure?
The spring does determine the fuel pressure. The spring is compressed during the powered inlet stroke and then the spring supplies the outlet fuel pressure.

Mick
Mick Anderson

I've never put in a new diaphram but I have had the pump apart. What you are experiencing could be related to the setting of the contact points relative to the shaft of the diaphram. Try screwing the diaphram a few more turns into the contact point pivot before closing it all up.

Chuck
Chuck Schaefer

Mike - If you will send me an e-mail, I can send you some pictures of the differences in the low pressure and high pressure pumps. They are different, but one can't tell the difference without taking the pump apart, which you have. The low pressure pump wasdesigned to produce a maximum pressure of 1.5PSI, whereas the high pressure pump for the MGAs produce 3.8 PSI. The low pressure coil will work very sluggishly, if at all, with a high pressure volute spring. There are also two different coils for the high pressure pumps that were used on the MGAs. One coil is the same length as the low pressure pump (even though it was a different coil), the other coil was about 1/2 - 3/4 inch longer than the shorter pump. The diaphragm for the long body pump is part number AUB 6099 where as the diaphragm for the shorter coil is AUB 6097. I don't think that your problem is an incorrect diaphragm in that a diaphragm for a short coil body will not reach the toggle on a long body pump and conversly, a diaphragm for the long body pump would be noticesably too long for the short body pump. For mor information on the pumps and how they work, see my article at: http://www.custompistols.com/cars/articles/dd_su_fuel_pumps_101.htm Good luck - Dave
David DuBois

I've just got back to this job after a couple of days off - thanks for your comments guys. Yes I knew about the diaphragm stretching procedure but what I hadn't appreciated was that with a much stronger HP spring (approx 6 lb compared with 2 lb for an LP spring) it is important to screw up the six housing screws much more tightly while the coil is powered and before switching off the current otherwise the diaphragm slips.

However, what I did was to get the pump going with the original parts and then replace a. contact breaker b. spring c. diaphragm and d. armature guide plate in this order checking, after each change, that the pump still worked. I got past a. b. and c. with the pump still working but when I put in the five new plastic armature guides (which seemed very tight) it stopped working.

So I've now got a working HP pump from an LP one but with eleven old brass guide rollers instead of the new plastic ones!

But I don't feel 100% confident that the pump will be reliable and I have a feeling there is something I've missed. So if you don't mind David (DuBois) I'd like to take you up on your offer of pictures of the two pumps - I'll get in touch via e-mail. Thanks again to you all. Mike

m.j. moore

This thread was discussed between 27/04/2006 and 01/05/2006

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