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MG MGB Technical - SU Fuel pump points

Installed a new SU fuel pump on my wifes 79 MGB LE, last year. The first drive this year ended a block from home when the pump stopped working and on inspection found the points totally blackened. Cleaned them and got it working again. But what is causing them to get to this condition in just 6 months of use last year?
Kim Rutherford.
K Rutherford

It sounds like a failed swamping resistor resulting in the points arcing and not being able to conduct the voltage necessary to operate the pump. Ray
RAY

What type of SU pump did you fit? There are three - the original capacitor quenched which can be fitted to either polarity; a later diode quenched in two flavours one for positive ground and one for negative. FWIW there is also an after-market pointless pump which may also be available in negative as well as positive.

The diode type for negative ground was AZX 1307, and the diode types can be distinguished by the plastic end cap *not* being stepped whereas the capacitor type had a stepped end-cap. I think the plastic tape round the end-cap was also blue on capacitor types, and either red or black for diode depending on polarity.

With all types the swamping resistor is only part of the quenching, the condensor/diode is the other part. Could be either or both of the swamping components failed. If the pump was a rebuilt and not a new one I'm reliably informed that often a burned out swamping resistor is often overlooked as part of the rebuild process. Your best bet is to contact Dave Dubois, a member of this and other fora, who knows these pumps inside out and offers a rebuild service - http://www.custompistols.com/cars/dave/dd_su_fuel_pumps_101.htm
Paul Hunt 2

Paul,
The pump was the capacitor type with the stepped end and the blue tape.
Kim R.
K Rutherford

OK, may be a faulty capacitor and/or swamping resistor then. Capacitor is easy enough to replace, 0.47uF of the highest voltage you can get to match the existing component, if that isn't marked with voltage. To determine whether the swamping resistor has failed you would be best off speaking to Dave, he may be able to give you resistance measurements. If you can operate the pump on the bench, and see visible arcing, that is significantly reduced when an alternative capacitor is connected, then that is probably all that is wrong. Earlier MGs only had the swamping resistor, the points did burn but they were a routine service item to clean and eventually replace, and with the pump on the firewall so much easier to get at.
Paul Hunt 2

New SU fuel pump with a capacitor for arc suppression??? I suspect that you mean a new to you pump or possibly a NOS pump. The pumps have not come with a capacitor used for arc suppression for many years. Regardless, it is not uncommon to find points that are blackened even though the arc suppression circuit is in good condition. There is always a certain amount of arcing of the points regardles of the method used for arc suppression. The tungsten contact of the points forms a film on them over time and without a small amount of arcing to burn the film off of the contacts, the film will eventually get thick enough to insulate the contacts. Kim, remove the pump and thouroughly clean the points with some 400 grit sandpaper until you have bright metal showing. You should do this every spring when bringing the car out of hibernation. This will remove any film that has formed over the winter so that the pump will run properly. The alternative is to have the pump converted to solid state or purchase one of the all electronic SU fuel pumps now available. Cheers - Dave
David DuBois

I considered the electronic SU pump, but my local Moss dealer advised me against them as he has had so many returned and considered them problematic. After cleaning the points with fine sandpaper the pump started working again, howeverI mentioned what had happened to my Moss dealer and he suggested I return the pump to him and he gave me a new one.
Thanks for all the info.
Kim R.
K Rutherford

This thread was discussed between 23/04/2007 and 24/04/2007

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