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MG MGA - Fuel pump assembly

I have 3 questions for the experts!
Which way round does the blue plastic diaphragm go on the exit chamber - it is dished - does the dish bulge into the cover? (I should have noted this!)
The repair kit gives 2 fibre washers and 2 O rings to fit the banjo connections. I had 4 fibre washers on dismantling. Which should go where?
The kit comes with a little blue "varistor" instead of a capacitor for the points. It is polarity sensitive but no markings! What to do - re-use the capacitor or try and find its polarity?
Art Pearse

Art - "Which way round does the blue plastic diaphragm go on the exit chamber"

It doesn't matter. I put them with the convex side out, but the pump will ultimately decide which way it wants the diaphragm - I see them both ways.

"The repair kit gives 2 fibre washers and 2 O rings to fit the banjo connections. I had 4 fibre washers on dismantling. Which should go where?"

If you had 4 fiber washers to start with, you should reassemble the banjo fittings with fiber washers on both sides of the banjo. You should be able to reuse two of the original washers. The 'O' rings are only used on pumps with inlet/outlet posts that are recessed for the 'O' rings.

"The kit comes with a little blue "varistor" instead of a capacitor for the points. It is polarity sensitive but no markings!"

The varistor goes either way, it is not polarity sensitive. Cheers - Dave
David DuBois

Thanks Dave.
I guess the blue diaphragm function is to trap an air volume behind it to even out the pulses?
Is the varistor better than the capacitor, and is its function radio supression or points erosion?
Art Pearse

Art - Yes, the purpose of the blue diaphragm is for flow smoothing of the pumps output.

The Varistor functions as part of the arc suppression circuit (in conjunction with the internal swamping resistor). As to whether it is better than the capacitor is up for grabs. I personally don't see how it functions as an arc suppressor, but maybe it does. I haven't looked at a pump that uses one with a scope to see how it functions. I suppose that I should do that sometime and report my findings. Cheers - Dave
David DuBois

The Burlen instructions contradict themselves! In one place they say the varistor is polar, in another that it is not! So I was confused. I installed it vs the capacitor.
When I installed the blue diaphragm, with the thin gasket and O ring, there remains a small gap between the cap and the body. I assume it is OK?
Art Pearse

Art - The small gap is normal. Cheers - Dave
David DuBois

Varistor is not polarity sensitive, nor is capacitor, but Zener diodes are; all were fitted at different times. The capacitor acts as the electrical analogue of the blue diaphragm damper. Zener & varistor both conduct to ground above a certain voltage, so limiting (clamping) the voltage of any spikes, which kills arcs. Zeners have one pointy end to tell you which way it gets connected - but I don't recall the code!

FRM
Fletcher R Millmore

This thread was discussed between 08/05/2010 and 11/05/2010

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