SU Fuel Pumps and a question about ignition points

[Tony Griffiths]

For those with an SU carb fitted, or a special - or whatever - and have an SU electric fuel pump fitted, I recently overhauled mine with a kit from Burlin like this: https://sucarb.co.uk/su-fuel-pumps-spare...-1985.html
A fiddly job, but easy enough if you take your time. One thing that interested me was that the kit came with a "varistor" (in effect a two-way diode) - though with no mention of what this did in the instructions. Easy enough to find out - it's to stop sparking across the points: https://board.mmoc.org.uk/viewtopic.php?t=76655
Could one of these "varistors" replace the capacitor on ignition points?

[Chris KC]

Um - I haven't a clue Tony, but having overhauled a couple of pumps using Burlen kits last year I'm interested - both because I don't know the answer, and because I don't believe there was one in either of the kits I bought (maybe you got a different kit? Or maybe they have rationalised their offering).

[Tony Griffiths]

Um - I haven't a clue Tony, but having overhauled a couple of pumps using Burlen kits last year I'm interested - both because I don't know the answer, and because I don't believe there was one in either of the kits I bought (maybe you got a different kit? Or maybe they have rationalised their offering).

Yes, my kit appears to be the latest - with revised instructions that have wiring diagrams for both a capacitor and 'varister'. The latter is available on its own https://sucarb.co.uk/supressor-assembly.html though it's stated as being out of stock. This is a recommended one from eBay: https://www.ebay.co.uk/itm/185903604781?...R-jnicOFZA

The SU website looks to have been designed by a 12-year-old on acid. Looks modern and clean but is difficult to navigate. The menu system is hopeless and the search useless - put in "6-volt fuel pump" and you get almost anything but. Finally, I phoned them about a different enquiry and was given excellent help.

[John Cornforth]

Hi

The simple answer is no, the ignition points condenser can't be replaced by a varistor (voltage dependent resistor).

To give more detail, the ignition condenser's main job is to temporarily store energy from the coil immediately after the points open, which is then rapidly released back as useful energy into the ignition system. A varistor would just turn it into heat, and reduce the spark energy. The condenser and the coil primary inductance form a resonant system. The resonance frequency is not ultra critical, and although the normal condenser value is 0.2 microfarads I have found only a small reduction in performance with 0.1 or 0.4 microfarads. A beneficial side effect of the condenser is a considerable reduction (but not complete elimination) of the arcing at the points.

I should say at this point that as I have never actually tried a varistor across the ignition points I can't be 100% certain of the above - maybe someone of enquiring mind could try it and report back !