AED mystery cut-off valve?

Hi everyone, I have a 3.4L S3 engine which I am transplanting into a Mk2. I don’t know the heritage of this engine other than it was supposedly “a runner” when I purchased it many years ago. Anyway, I’m now at the point of actually prepping the engine for a recondition. Does anyone know if the valve in the fuel line to the AED was ever standard? I’m guessing not; fitted by a PO to bypass the AED?
PS the oil filter is changed

so it will actually fit a Mk2 engine bay.


I don’t think so. My 1975 4.2 XJ6C had one when I bought it, I restored the engine and AED and everything worked perfectly for over 10 years, but then the AED would not shut completely off, resulting in very rich idle so I added a cut off valve in the AED fuel line last year. It works well for now. YMMV.


I’m not familiar with how it works but wouldn’t it also need an air shut off, or is that negligible off idle?
What powers such a valve?

Thanks Pekka, so this seems a common modification

Hi David, the one on my engine is 12V. It’s normally shut and +12V opens it. Of course I have no idea how it was wired to operate when it was installed.

Hi David,

AFAIK the original design is all mechanical / bi-metal etc. There are no electrical connections.

The PO mod was a fuel valve, and now after long consideration I also decided to ad one (I wrote about it about a year ago) where I put an illuminated (when engaged / powered) fuel switch which allows fuel to enter the AED when powered on, and then when the switch is off, there is no more power to the small solenoid valve and all fuel is cut off the AED.

The AED is supposed to pass some air also when it’s all warmed up, but not fuel.

Just removing of blocking the AED is not a good option IMO, I tried and it was very hard to get the car started and running without the AED, mind you I have 2" SU HS8’s, original to my Jan. 1975 XJ6C, it may be different for those who have Z-S’s or SU HIF6’s. Or then not very different as many E-types also with Z-S’s are often a bit hard to start cold, unlike the triple SU HD8’s which IMO always start immediately with the manual choke pulled all the way (and my 4x Z-S V12 as well).
Many also adapt a manual choke in the XJ but I didn’t want to take that route, especially since my wife and daughter also occasionally drive the XJ6C. YMMV.


If it’s passing air anyways I assume it doesn’t matter. I was a bit worried. I like my earlier ASC, it is not very sophisticated but I can hear it running and it starts immediately (there’s only a small period after shutting off where the car is a little lean and it needs getting used to when suddenly at a light everything goes very quiet and smooth). And I can shut it off manually at idle If I want to, just turn the ignition off and on again. A simple on off affair.

This would go the same way with the electric switch but likely feel more sophisticated. A temperature sensor would be nice, something like a fan switch that connects to the AC and shuts off at 50° when that turns on

Hi David,

Agreed the ASC aka. “the hisser” is a MUCH simpler device. I have only ever had one, it’s on the 1948 MKIV engine that’s inside my 1950 MKV DHC.

Mine is so early it does not even have a blow-back valve, which is probably why a PO has put a small switch (looks period correct) below the streering wheel for powering it.

Once it’s off, it’s off as the spring closes the whole passage the solenoid opens when powered. The reason the switch makes sense to have on a very old car / engine is that in the original design, when you have ignition on, that wire going to the small solenoid (with bakelite covered small thumb nuts) will always have 12 volts (in my case -12V as the car is positive ground/earth) and I agree with Ed Nantes that it’s not always a great idea to have a live wire right next to the carbies.

The AED is great when it works perfectly, easy and always starts without any hesitation, BUT once it starts to misbehave it can really drive one nuts! :smiley:

So I thought I can live with that switch, and I used an amber one with a light as I thought it would also catch the eyes of any lady drivers reminding them that something is on and the amber light is a little bit alarming, but the car will not start cold without it.


PS. Here:

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I love simple tech, I lost the thumb screws and not sure about a blow back valve; the little spring is only there to keep it closed, the manifold vacuum does the real work (this is why turning the ignition off and back on quickly turns it off; the disc falls shut and the solenoid is not strong enough to pull it back up unless you push the throttle open).
I’d not worry so much since the distributor is in the area too and the ignition wires can create quite a good light show (and so I’m waiting for DHL who will bring new wires, cap boots and so on in the next 5 minutes).

Back in the day I’d have preferred the ASC but I’m driving an old car because it is old and mechanical. The added switch is a good idea either way. Looks good!

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Thank you both… I’ve read a few posts about the dreaded AED and it seems the best option for now, assuming I can get it to work, and that there’s enough room for the hot air pipe in a Mk2 engine bay.
Cheers, Phil