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MG MGA - Fuel Pump Diaphram

Last night I spent a few frustrating hours overhauling my fuel pump. Biggest frustration (after picking up the little plastic guides off the floor for the 100th time) was that the diaphram was not flexible enough.
When fitted the new diaphram would push up the inner rocker making the outer throw over and no longer make contact to the contact blade on top of the pedastal. It seemed the spring was not powerful enough to push the diaphram to the correct position.
Refitting the old diaphram, which was still in good condition, got the pump working again but it got me wondering if there is a good way to stretch the diaphram?
Ideas welcome.

Neil Purves

Neil - There is no need to stretch the new diaphragms when installing them. Are you sure that you got the correct diaphragm? There were two different pumps used on the MGAs. The original has a longer coil housing, that uses a diaphragm with a 3" long spindle. The later pumps have a shorter coil that use diaphragms with a 2 3/8" long spindle. Often, when ordering fuel pump parts for a MGA, the vendor just assume that the pump uses the diaphragm with the 3" spindle. Cheers - Dave
David DuBois

David makes a good point about the diffent diaphrams. However, the manual does recommend stretching a new diaphram and shows a special tool used to do it. I believe it also says match sticks can be used in lieu of the tool. I've not needed to install a new diaphram (they rarely go bad) but usually dismantle the pump when installing new points to set the throw-over. Did you compare the new and old diaphams/spindles? I am curious about the "little plastic guides" you mention. I just did a pump and I don't recall any plastic guides. Could you explain?
G T Foster

Gerry - If you are referring to the shop manual for the MGAs, keep in mind that it was written over 50 years ago. With today's material used for diaphragms, there is no need to stretch the diaphragm when installing a new one. In fact, the shop manual put out by Burlen Fuel Systems (maker of all things SU today) does not mention stretching the diaphragm when assembling a SU fuel pump. I have been restoring pumps for over 30 years and in the last 20 years, I quit stretching the diaphragms as it either didn't make any difference, or in extreme cases would cause the pump to become unstable. None of the other people who repair the pumps that I have spoken to stretch the diaphragms either.

The plastic pieces that Neil refers to are the diaphragm centering devices. Over the years there were three different devices used to center the diaphragms. The first was the 11 brass discs with a hole in the middle (the ones that wound up scattered on the floor the first time any of us disassembled a pump). The second was four oblong disks held together with a plastic strip between three of them. The third is 5 plastic figure eights that fit very tightly between the diaphragm armature and the ID of the coil housing. These figure eights work much better than the previous two at keeping the diaphragm centered. Cheers - Dave
David DuBois

David, you have solved my problem, thanks. The shaft on the new kit is longer than the original, it is therefore pushing up the rocker when the housing is screwed together.
As the old diaphram is still working well I shall leave it in and hope I remember this issue the next time I come to repair the pump!!

Neil Purves

I have an original fuel pump that was converted to a magnetic reed switch instead of the old points by Dave Dubois. It has performed flawlessly. I've also followed his advice and installed relays for the headlights and brake switch to reduce the current that those old Lucas switches carry. As far as I am concerned he is the "Guru" when it comes to fuel and electrical issues for these old cars!
Tom Heath

Thank yo for the good words Tony. It is always nice to hear that, if somewhat humbling. Cheers - Dave
David DuBois

This thread was discussed between 19/08/2009 and 23/08/2009

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