XK120/140 H6 mixture adjustment

Just out of interest, how do folk generally adjust the jet height (mixture) on their H6 carbs? From above, or via the wheelarch, with the screwed access panel removed? The latter is a lot easier, but means doing the job with the front wheel removed, which would not be possible on a rolling road tune. It’s very hard to access the jet screws, let alone judge the number of turns accurately from above.


I don’t know about the 140, but on the 120 I find with the caps removed I can lie on the garage floor and look up to adjust the exposed screws. I would note that I have a horizontally mounted vertical splash panel behind the front wheel having replaced the angled factory item, this does open up the area below the carbs a bit better access wise.



The best way for me is still from above, resting on the wing/fender (with a blanket to prevent any scratching of the new paint). Of course the air-filters have to be removed as well as the cap at the bottom, but that’s it.
The adjusting screws Jaguar 3158 (normally) have a “cross slotted” end. I made a tool by cutting the Pozidrive tip of a short screwdriver to create a short cross that perfectly fits in the adjustment screw. See picture.

This tool creates more stability when turning the screw up and down, and you can count the number of (quarter) turns exactly, as you can see the screw driver tip from the position on the wing/fender. It worked for me every time.

Bob K.


Thanks both - that’s a good tip, Bob (joke). The screwdriver always slips out of the slot when I’m doing this. I’d also think about painting a really obvious white stripe down the top (in working position) of the handle, and down the side. Each turn is clear to see then.
I’ll sacrifice a screwdriver later today.

Next question - how on earth are you supposed to refit the air cleaners? I try to do the frontmost one first, but this morning it took me over an hour to get the first bolt started. There just isn’t room, and from underneath is also hopeless.
Is there a better air filter option available? It wouldn’t look as good, but these are an absolute nightmare to fit, and not very good as air filters, either.


Your idea regarding painting a stripe on the screwdriver mirrors exactly what I have done. I have a specific driver, one which I ground down to give me a thicker tip that more readily fits the mixture screw, then painted a mark in white on one side to facilitate the rotation count. Agreed, the front pancake filter cover is a regular pita. Even on my aftermarket pair using foam inserts (I haven’t run the original remote in front of the radiator with it’s ducting under the wing since first purchase back in the sixties) it is not a task I relish.



Fitting the AC Air Cleaners can best be done as follows (at least for me).

First put the two screws through the holes from behind. Normally they will stay there but you could also experiment with a 1/4 " longer screw.
Only then take the AC Filter in your left hand, still at a distance from the carb flange, just touching the end of the screws. With your RH fingers it will be possible to get the first two turns of the RH screw in the threaded insert of the filter, as you can still see where that hole is.
Then switch to the LH screw, holding the Filter in your RH hand. Once both screws are in, you can try turning the screws with your fingers as far as you can, before changing to the spanner for the final fixation…

Bob K.

I really struggle to get that first screw in, though. I can only hold the filter at the rearmost edge, as the body is too close at the front. The screw has to be positioned on the end of a magnet probe and pushed in with a long screwdriver. The float chamber is in the way, but can’t be moved… The only way I can find to get the thread started is to get a finger down to the screw head to hold it in position and try to twist the filter upwards to get the thread started. There is no way I can get fingers in to turn the screw - just one finger, which I can sometimes flick one flat at a time, but it’s not great. A pair of K&Ns would be a big improvement as the backplate could stay on, but I don’t think they do any that are offset towards the rear. They would also do a much better job of filtration.

Sorry Roger I had my XK 120 in mind and now remember that especially the XK 140 FHC was much more difficult due to the different body design.
The XK 120 is much easier in that respect.

Bob K.

I’ve got a feeling the 140 engine is further forwards than the 120, with the same panel structure.
I’m thinking about K&Ns, but realise there’s some surgery involved. There’s an old thread where someone modifies the front one to work, but his car is LHD. Mine’s RHD, so the brake fluid reservoir is a problem for the rear carb, too.
Does anyone have any thoughts on the advantages of changing to K&N type, or are the wire strainers considered good enough?

I adjust mine from above by feel.
That idea of a new lease on life for a worn out stubby Phillips is great; I’ll do that next time.
I live in a part of the world that is not all that dusty, so I am running my 120 with the original remote wire mesh filter.

On that basis Rob, I should probably worry more about filtering rain or mud - not much dust here, either.

@rsk55 Hey Roger, I did the K&N on my LhD 140 OTS. Like you mentioned I did modify the front carb. Doing the modification is pretty straight forward and after doing it I know there is room for even more movement of the filter housing.

As for filtering, I use K&N on all my vehicles not only for the best in the market but it is a one time buy because of the washable media. Good filtration is one reason modern engines last longer.

Thanks Dan. I take it you used the 56-9311 filters?

@rsk55 Roger, yes I got them from Terry’s. They had best price and in stock. I used 14ga stainless to make a new backing plate but galvanized or plain will work too. You can move the mounting and intake holes anywhere inside the filter element to suit what works best for you.
Good luck, Dan

If you decide to stick with original equipment, one “trick” I employed to overcome a seemingly impossible fastener placement on my triple SU 420G air filter

What I eventually did is grind down the top of the bolt head so that is was 1/2 to 3/4 as “long” as before…ie ~6mm to 3-4mm

This gives the vital 2mm (1/16") to allow the head to somehow squeeze in enough to get started

These bolts are not torqued hard, so the thinned head is of no consequence

You should be able to use a spare old Jag bolt if you wish to leave the original fastener unharmed, its a common size, if not a hardware item can be the experimental sacrifice

I have implemented this solution in other applications where appropriate

It may not be appropriate for your application, a photo would speak 1000 words