SU Fuel Pump XK120

yes Dave, there are cautions about working on fuel pumps and how they ground, and sparks with fuel around…sorry that happened…wow. Disconnect battery is the rule. It is good to have a battery kill switch anyway. I bet someone has a second pump all hooked up and just a switch or two to enable it, and disable the other. Nick

As an alternative to the high cost of a new square body fuel pump, used pumps can often be found on eBay for a bargain. They will need to be overhauled, but the parts are not that expensive.

@MarekH (here on Jag-lovers) has created a circuit board for the SU fuel pump to make it solid state and more reliable. His circuit board fits perfectly under the plastic cap and comes with a full set of instructions. MarekH makes the circuit board in either positive or negative ground.

I like having a ready-to-go spare fuel pump in the boot to avoid the potential of having to cut the copper fuel lines to install a makeshift aftermarket pump just to get home.

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Mike, would these possibly also work with the SU-licensed period knock-offs made by Harting in Germany?

Christopher, I don’t know. Hopefully, @MarekH will chime in.

You fellows do know I trust that the later, much cheaper, round-bodied SU fuel pump uses the same two bolt mount pattern and takes the same line fittings as the square-bodied fuel pump? It will bolt right up to the XK120 frame with zero mods. Yes, not original but it is hidden under the floor after all.

Just picked up the car yesterday with the new SU fuel pump fitted - working fine. Turns out, the 5 year-old one that failed was, in fact, a points type (not electronic as I had thought I’d specified). Apparently, the pump is full of fuel, so obviously the diaphragm has ruptured. I’ll know more when I take it apart.

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Mike

“………I like having a ready-to-go spare fuel pump in the boot to avoid the potential of having to cut the copper fuel lines to install a makeshift aftermarket pump just to get home…………….”

Cutting any fuel lines is not the best approach, neither is disconnecting the lines or changing the complete pump.

Some 99% of SU pump failures are not caused by valve issues, that leaves the main coil body, all the electrics including points and the diaphragm. The approach is to have the main section of the pump with diaphragm and rollers available. Any pump trouble, you need one screw driver, remove the six body to pump chamber screws and remove the main body, refit your rebuilt body and diaphragm, fit the six screws and drive off.

No need to disconnect any pipes, and on twin tank/ pump installations, the spare body will fit either pump. One of the tricks I learnt during many attendances at S.U Service School, Cowley, England.

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Excellent advice, Mike!

I’m wondering if my liberal use of “Dry Fuel” and also Sta-Bil fuel stabilizer when I was chasing the elusive steady-speed misfire (eventually cured by a distributor rebuild by a specialist) which developed after the winter, had something to do with the fuel pump diaphragm failing? Or, maybe it was something to do with Ethanol in the fuel, though only 5% is the norm here in the UK.

Using the later round body pump I agree the bolt holes will line up but the pump will not sit square unless you make a round hole in the frame surface in my 140 application. IIRC the 120 had it on a bracket inboard where 140/150 twin pipes would conflict. I wonder if putting in a second pump would require the fitment of a check valve to prevent loss of pressure back through the first. This might depend how the first pump failed.

I don’t know that ethanol would cause a failure as we have had 10% ethanol here for many years and it has not affected the pump in my 120.
All SU pumps have a built in check valve.
If you really want a backup you could put two in parallel, or another idea might be to use a double pumper as used in the XJ12, which is basically two pumps on one round body, working in parallel.

Apologies for being late to the party, but I have only just seen this. (J-L system software has been asking me to “turn on notifications” for a while and this is probably why.)

The circuit board ought to work for any pump with the same five bolt pattern sitting on the bakelite end which has two cutout knotches in it and which uses the same bell shaped points. I append one of the pictures of the circuit board from the fitting instructions. Screw “B” still holds those points, but the circuit board simply sits on top of them like a big washer.

kind regards
Marek

Well, a spare fuel pump or parts thereof is something else to add to the list of spares when touring.