Square Body Fuel Pump vs Round & Solid State vs Mechanical Pumps

the square body pump is the original style for a XK140. Is there any disadvantage in going with the recommended replacement Round Body SU Pump for a cost savings?
Are they Solid State Pumps more reliable now then the original mechanical style SU Pumps?

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I have a round SU fuel pump in my XK140, I first tried a solid state pump.
I had a square pump with the car, that was defect ( just rotted away )
But somehow it had problems sucking up the fuel, and I did not like the sound anyway.
On my S-type and XK150 I used a Hardi fuel pump, looks just like a SU, and very reliable.
And makes the same sound as a SU.

You can now get the correct square body SU pump with the points replaced with solid-state electronics. Working well on mine so far.

I converted my LCS square body with a transistor and diode and resistor, and it’s been working fine for 20 years.

just be sure that whatever the pump…that fuel pressure is the quite low 2.7psi specified by SU. Even 4 can be too much. Many modern fuel pumps deliver much higher pressures. All the pump does in the SU is maintain the float level.

…and too high a pressure will damage the carburettor float valves. As Nick says, low pressure is essential.

I’d like a drop of what you’ve been drinking, please. It seems to make the impossible commonplace?

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Speaking from the perspective of the XK120, a new replacement SU round body fuel pump will bolt right up to the mounts on the frame and the fuel line fittings will mate properly. Slam dunk, and since it’s hidden under the floor boards you need not be concerned about sniffing commentary from cruise night lawyers. However, the polarity of the modern replacement must be positive ground, unless of course you’ve done an alternator conversion.

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don’t toss that old square pump…the core body can be rebuilt, or converted…and has value.

I have bought a few unrestored square bodied pumps from auction site, with time and patience you can get them for around $100US, so not cheap!

I have rebuilt them but not replaced the rebuilt round ones I put on to replace plastic ones that came with the car.

The square bodied valves look more primitive than the later ones, but I don’t know if this makes any difference.


Found my pix of the transistorized SU LCS pump.
SU fuel pump 009

I’ve done about a dozen of these, LCS and double pumpers, used to buy these parts at Radio Shack, but they’re gone now, so I’ll have to try the internet to find more. I have one or two more yet to do.
This one is on the '38 SS.

Were those brass extinguishers ever fitted by the factory? I have a couple if anyone knows what they’re worth? First time I’ve seen one in a photo.

Not by the factory, but they were a popular motoring accessory back in the 1930s.

Definitely an SS accessory then, not D-Type!

Dear all,

Let me please wake up this old thread.

I took the XK120 for a long ride with my daughter yesterday. After after roughly 180 Km, in rather warm spring weather, we sat 5 minutes in traffic.

The idle became suddenly irregular and the engine quit. After an instant of panic I noticed that the fuel pump did not click. One minute later the car started without problems.

Coming back home, after a total of 300Km and a fast (110-130 Km/h) highway drive of 100Km, we had the same situation in front on the garage door. Since it was all quite, I manually opened the floaters with pins, and the pump did NOT click, did not start (hence it could not be vapour lock, right?)

I suspect that the pump (square body SU) is on its way out.

My intention would be to send it to a rebuild service and have anyway a cheaper round body, while I am waiting and as a spare later.

Any thoughts on what can be causing the (hot) erratic behaviour of the pump?

It has probably 3 years and 3500 Km and despite no leaks, it seems to click a lot when the tank is empty (so I suspect a suction leak).

Many thanks.


If the pump does not click, the highest chance of the problem is points arcing
(unless you have diode or transistor protection)

Once the pump is removed, erosion of points can be observed with 10x magnification.
The points are titanium, (or tungsten) so a very fresh file is needed

I got several going by doing this, the pedestal can be moved aside by removing one screw to access the lower points.

I added a diode, and that eliminated the arcing

It was extremely surprising how badly eroded the points were, but shouldnt be, the arc is very visible, the pump pulls about 4 Amps, and I believe the back EMF can be hundreds of volts

A complete rebuild kit is about $100, but not needed before trying to file the points

In my opinion, it is not hard to rebuild one, except getting the steel screws out of the alloy body can be very tricky ( only needed for a rebuild)

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Thanks so much: why would the behaviour be only noticeable when hot? There is a shop here in The Netherlands that has introduced an optical switch that does away with points. I am considering it-

That I cant answer, maybe you do have vapor lock, but I personally doubt it,
unless the weather was very hot, or some other issue

I got 4 pumps in a row going by doing this simple procedure, and each one had pitted points, which could only be seen with high magnification

A TVS diode is best to fit as they are not polarity sensitive, but do you think I could find the 10 I got off Ebay?..NO, so I fitted 1N4007 diodes, which are polarity sensitive, but do a similar job

You do have to remove the pump, but the cost of the job is about $2 for the TVS diodes and file points, if it does not fix the issue, then you can spend $

This procedure worked on LCS and AU303 pumps, and a Lucas copy

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Scroll up and you will see a round body pump on my 2-1/2 Liter SS. The 3-1/2 Liter SS and Mark IV had two of these in parallel because one did not have enough capacity. That was why they went to the square body “Large Capacity Single” pump with Mark V and XK120 and subsequent models.

There have been quite a few discussions on this forum about SU fuel pumps.
I like the transistor conversion scheme and have done about a dozen.
But my local source of transistor parts has gone away.
Next time I will probably try the Hall Effect version marketed by Burlens.

Check that the diaphragm in the pump is flexible. They get stiff after many years.

But before you go that route you might look at the fuel line filters for blockages. There is one in the tank drain plug, one in the pump, and one in each carb in the float bowl.

Many thanks. On the topic of temperature: would it be possibly that as everything get warmed up, the porous, rough, arc-eroded surface simply becomes sticky?