Carburettor Needles for a fast road XK120 setup

I think I need some tuning advice…

What needles would you recommend for an xk120 3.4 engine with:
• Gas flowed head,
• inlet valve, - 1 7/8”
• exhaust valve, 1 5/8”
• JAG2 cams from Kent Cams (fast road)
• lightened flywheel



What carbs do you have HD6 or HD8 ? doubles or triples.

Standard xk120 setup…

Impossible to say with any accuracy, too many variables - you can only guess at a starting point, and go from there. Any decent rolling road operative with SU experience will be able to set you up. I can highly recommend Peter Baldwin in Cambridge, who’s been doing this for 50 years now.

Back in the sixties, I believe Pete drove the twin-Jag 4-cylinder-engined Mini on a few occasions…

One in the front, one in the boot. Those motors obviously ran on DCOE Webers, not SUs. Although now in his late seventies, I don’t believe anyone can tune SU carbs better than Peter.

1 Like

Oh, maaan, would I love to see one of those, today!

It boggles the mind to consider just what it would take to synchronize two sets of gearbox and clutch linkages.

1 Like

There have been a few twin-engined Minis, the best known being the ‘Twinis’, powered by twin Mini Cooper engines. The project was dropped when John Cooper was severely injured in one when one of the gearboxes seized.

Sorry OP, this has gone very OT…

It still exists - that photo was taken on the Mini Cooper Register stand at the NEC Classic Car Show quite recently.

To return to the question, my suggestion would be:
If the carbs were rebuilt professionally by someone like Burlen, ask their advice for a starting point and fit those. Then, if the engine is out of the car, take it for a dyno session, or if it’s in the car, a rolling road session. With those mods, I wouldn’t want to drive it too much or too hard before getting this done as there are so many variables we don’t know (cam spec, CR, flow rates, exhaust flow etc,). I would take it to someone who knows SU carbs inside out. An old-school racer like Peter has drawers full of needles and jets in his rolling road shop and will set up for your style of driving.

My memory’s not what it was, but I’m sure I remember Pete telling me about a one-off one-litre twin cylinder XK engine he was involved with back in the late sixties…


I guess this is “unexplored territory” for the average XK enthusiast. The description of your head is way beyond that of a good C type head and that is where my knowledge ends. Like Roger mentions: also Jaguar switched over to Webers at a certain point of engine upgrading.
I can help you with the data on needle comparison for the “best C-type” head Jaguar made for the XK 140 (after which the B type heads were introduced) all with H6 Thermo carbs (so not H8).
Another question is what you want to do with this engine: touring or racing? Various books indicate what needle section is used for “cruising & pick-up” and what section for “top speed”. There are various needles to choose from to optimize your objective.
As an example, see the comparison below, but once more this stops with the WO2 needle for the XK 140 C-type head. The graph shows the “measuring point” of the needle on the horizontal axis (indicating the position of the needle relative to the jet) and the diameter of the needle (in mm, sorry…) on the vertical axis indicating the amount of fuel passing the jet.

Bob K.

Bob’s right - and you also have to take any ‘period’ suggestions with a pinch of salt as modern fuels are nothing like what was used then. With a modified setup, you can only do this reliably on a rolling road (or dyno) if you want to guarantee no damage to the engine components. I will be rebuilding my 140’s ‘C’ type engine as close to stock as I can get it, but it will still be going nowhere much until Pete’s weaved his magic on carbs and ignition.
Speaking very personally, I like to keep these motors close to stock for the most part. Even an XK fitted with a seriously breathed-on, fully be-Webered and hot cammed motor will not be particularly quick by modern standards so I prefer to keep the components less stressed and enjoy the drive. That said, I do big mileages in my old cars so they do get used.
Or at least I did last year… this year I seem to just look at them, sitting there doing very little.

Thanks Bob for getting this thread on track :slight_smile:
Being Polish, metric system is fine with me and I actually prefer it :wink:

I usually try to upgrade my engines to slightly better than stock, to get them to fast road specs…

I guess I have to buy a few sets and experiment.


Hi Tadek,
I have similar specs on my rebuilt engine and have 9:1 compression Mahle pistons. You did not say what your compression is so the following may not apply. However, I started running with the original WO2 needles and they were way rich. I determined that with a dual air fuel monitor that a few of us jointly purchased. Plus anyone following me was gassed out by the unburned fuel, and they told me that in no uncertain terms. So I referred to a book on SU carb set-up and found the standard progression of needles that Jaguar used on 3.4 liter engines and looked to see what I had in my collection.

The progression from leaner to richer is listed as DG,TM,TL, SJ, CI, TU, WO3, WO2, RF, RB, RC. Red carb springs were used as standard. I had most of the leaner needles and chose SJ and installed them. They are a huge improvement and I am currently still running them. The engine idles well and revs easily and quickly and has great power. I have yet to test the air fuel ratio but am quite happy with the way it is running. And nobody following me is getting gassed out anymore.

This has been my experience so far and I hope it helps.

1 Like

Just to mention that an experienced SU tuner will polish the standard needle profile to suit your exact engine. It’s very difficult to check the mixture right through the rev range at load without doing it on a rolling road. One alternative, which I use for checking rather than initial setup, is to weld a bung into the exhaust collectors and use a wideband O2 sensor, driving the car in real time and under real load conditions. I use an Innovate, which will record the test for downloading later. This will give you the AFR over the whole throttle opening range, but with SUs interpreting this into modifying the needles is still quite a skill. It’s more straightforward with fixed-jet carbs.
I doubt you can do much about unburnt HCs for those following you, though - I’d have thought that was down to the cam profile.

I was (years ago) also a bit surprised to see how much richer the WO2 needle was in comparison with the SL needle, that was still prescribed in SB 165 dated Feb 1955. Don’t know the reason for the introduction of WO2 (according SB 168 of June 1955), whereby a much richer mixture is provided both in “cruising” revs as well as “top speed” revs . Or was it done in an attempt to obtain a true 140 mph top speed for the XK 140 (in stead of “just” 130 mph)??

Bob K.

Hi Tom,

Yes, Of course I forgot to mention - the CR is 9:1 with Mahle pistons.

Great advice, I will have to experiment, trying 1st the SJ as you recommend.


Another good—though time consuming–method is the ‘clean cut’ way.

Drive at a steady state, at any given speed/ throttle position, see a safe spot to pull off, then simultaneously declutch it and shut off the ignition.

Coast to the side, and pull every other plug, and see how they ‘read.’

Not as accurate as with old tetraethyl gasoline, but can still provide a rough idea of too lean/rich.

They could have quite a ‘step’ in specification, Bob. The issue with SUs, though, is that the change in mixture can be weak in one area yet rich in another on the same needle - that’s where the really clever tuners come in. Webers are a lot easier.

1 Like

I also contacted Guy Broad though a friend and he has suggested, with such spec engine,

"- using standard 120 H6 carbs WO2 needles.

  • With open trumpets on the carbs WO3.
  • Using 2 inch carbs UVO needles."

I thought I would add the info to the thread.


1 Like

The richer needle recommendation was based on customer complaints of “spitting back” thru the carbs during lower-to-moderate rev acceleration…in other words, under heavy load. Read that in one of my books.