XK 140 engine idling fast when hot

Hello there,

I’m new to the forum having been given my Dad’s XK140 last September.

It has electronic ignition and starts and runs pretty well from cold, however when hot, and particularly when started hot, the starting carb doesn’t seem to disengage. As a result it idles at 1500rpm and is challenging to drive in stop-start conditions.

Does anyone have experience of this, and do you have any ideas as to how I can fix it?

I’ve seen forums where people suggest adding a switch to disable it, but before I consider doing that, I’d like to try and get it working properly if possible.

A general description of how the starter carb works would be really helpful. For example, I don’t know how, and by what mechanism the solenoid is switched?

Many thanks in advance,


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Dear Phil,

Yes: you will need to check a few things:

I had the same issue just after buying the car.

  • Throttle pedal bolt: it’s near to the exhaust and when warm it will not let the pedal travel fully back
  • Throttle linkage, clean and oil all joins
  • Throttle shaft(s): check that you have either the flexible spiral adapter or expansion joints between the shafts: when hot the, shafts expand and do not let the butterflies close
  • Clean the carburettor dashpots and while you are at it the butterflies ( with carb cleaner, brake cleaner is very aggressive)
  • Check for tight manifold to body and carb to manifold boltes, vacuum leaks may get worse with temperature
  • Check any other potential vacuum loss, using short sprays of brake fluid (not too much, it’s aggressive) you may find the spot, as the idle speed changes.
  • check and grease the shaft-to-throttle-body spindles, with age they have some play and when hot they can be the source of a vacuum leak. In case not too bad, heavy grease helps.
  • Last but not least: check spring tension on the throttle mechanism

In case that helping the shaft close with your fingers lowers the idle, you can be pretty confident that a good clean up of the mechanical couplings, and lubrifying the shafts, as well as using flexible joints will solve the issue.



The Auxiliary Starting Carburetor is a solenoid operated device located between the carburetors. It is activated by a temperature controlled Otter switch located in the water manifold. When cold the switch is closed activating the solenoid and the unit injects a rich fuel mixture. It is very difficult to start a cold XK engine without it. Once the engine warms the Otter switch opens and deactivates the ASC. If it does not the engine will run very rich at high idle speed. When activated the ASC will make a pronounced hissing sound. You can manually deactivate it by disconnecting the Otter switch wire. Another poster mentioned spraying brake fluid. Do not do that as it is not combustible and is an excellent paint remover. If the ASC is not active and the engine is idling fast make sure the butterflies are fully closed. You could also have an air leak (brake boosters are a common culprit). Finally, the engine timing could be too advanced.
Pat H

Many thanks both for your prompt replies.

I have checked the throttle linkages, and all seem to have appropriate expansion mechanisms and are moving freely. The spring is pretty strong and the linkages continue to move freely when warm. I’ve oiled them also as a precaution.

What seems to happen us that when the starter has cut out (the pin is up and the hissing has stopped), a blip of the throttle causes it to cut in again until I disconnect the 12V connection.

Any idea what could cause that?

I have a video but don’t seem to be able to upload it. Hopefully this YouTube link works?



Welcome Phil.
Here is the Otter switch on my XK120. Yours is the same switch held in by 3 screws and the location is similar, although you have a different intake manifold.

It is supposed to disconnect at 35 degrees C or 95 F.
It sounds like yours is failing to disconnect.

The pin on the starting carb drops when the solenoid valve opens, and is an indication that the starting carb is working. High manifold vacuum at idle sucks on the valve face and prevents the valve from opening, but when you blip the throttle the vacuum drops and then the solenoid is strong enough to open the valve. When you pulled off the wire on the solenoid, the valve closed and the pin popped up again. All that is normal.

You just have a bad Otter switch. (Otter is the brand name, usually marked on it)
What country are you in? LHD or RHD?

Sorry, mis-read your post. It’s clear cut. I would just ground the misser with a switch under the dashboard. Mine works, but I find it too persistent, as it usually cut at something like 40°C, much too high for my taste.


Thanks, that makes a lot of sense. It does seem to shut off correctly when the engine first starts from cold, but then once the engine bay is pretty warm and has been running at temperature for a while it opens again and seems to stay open.

I’m in the UK, RHD, although my 140 was imported from the US and converted 20 years ago - pretty sure it’ll have RHD parts? Would a photo of the engine bay help?

Thanks again!


How’s this?


8305. XK & MK2 ‘Otter’, Choke Thermostat Switch & Gasket . C2474. C2475


](8305. XK & MK2 'Otter', Choke Thermostat Switch & Gasket . C2474. C2475)


Hi Rob,

Mine looks to be on top:


Thanks Luis,

I’ll probably replace the Otter switch and also fit a switch on the dash so I have the option to switch off early if needed.


While not pretending to understand why it acts up the way it does, my starting carb works normally about half the time - that is, the engine starts up immediately, idles at 1500 or so then a minute or two later the revs drop down to 700 … or they don’t, in which case I blip the ignition switch - rapid off/on - to close the starting carb. It works every time. I struggle to imagine how the otter switch is the problem - if it was remaining closed despite the rise in temperature then blipping the ignition switch should have no effect … or is my logic out to lunch?

Thanks Nick,

I tried blipping the ignition key this evening and it does ‘reset’ the solenoid as you suggest. It does return to over-fuelling however so needs doing often…

I unplugged the 12v to the solenoid on the way home this evening and it drove beautifully. Have bought a toggle switch and some spade connectors so that I can switch out the 12V to the solenoid for the time being.

The wiring diagram shows the otter switch merely grounding one side of the coil when closed/cold, so I do wonder whether it’s not the bimetallic switch itself, but a breakdown in the insulation within either the otter switch or the solenoid, creating a partial route the ground and therefore energising the solenoid?

Any thoughts?


I suppose if your toggle switch breaks the connection between the otter switch and the solenoid and the phenomenon persists then you will have isolated the problem to the starting carb itself.

Phil, correct, the Otter switch just makes or breaks a connection to ground.
The other side of the solenoid is live with the ignition coil SW or minus side if you still have the car positive ground.

Nick, yours sounds like the Otter switch is faulty as well.
Someone has posted on this forum about how to take these apart and fix them.

I will add that to the list … once I replace the track rod end bushings.

Actually that’s a good point, I hadn’t thought to unplug the ground side of the solenoid and see if the issue goes away. Will try that in the morning.

Thank you,


Connect an ohm meter between the Otter connection post and ground. I should show 0 ohms when cold and open circuit when hot. The Otter can be disasembled and adjusted by bending the element

Just wanted to report back that I had the Otter switch changed along with a new water pump (which was seeping) and all is good now.

Many thanks for your advice,


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