Converting to H8 carbs

I have a 1956 XK140 SE. I am thinking of converting to H8 carbs. I hate the automatic choke on the H6. I know the coil mounting bracket, the vacuum advance on the distributor and the air cleaners will have to be changed. Does anyone know of other necessary changes? The conversion kits come with the right linkages.

The original H8 setup on an XK 140 included a mechanical distributor (no vacuum advance); a choke cable; longer mounting studs; two insulators for each carb; larger openings (2 inch) at the carb to intake interface; a larger internal passage between both sides of the intake manifold; a blanking plate to replace the Otter switch; a special coil mount; a unique fuel intake; and special pancake air filters.

I have a manual switch hidden under the dashboard to operate mine. In summer I am shutting it off by the time I pull out of my driveway, and I don’t use it again for the rest of the day.

Why do you want to fit H8 carburetters???
H8 carburetters were and still are a fairly unsophisticated 2" SU carburetter, that was intended for certain competition use, but not all, where the over-fueling allowed some performance gain at high rpm, but did nothing for low and mid range rpm. Indeed for a road car, they are distinctly problematic at their usual operating range. The only benefit that is shared by many, is the WOW factor of their appearance - they look fantastic when you open the bonnet, as in when they are not actually working… I love the looks of them, thus have a complete/original XK140 H8 set-up, mounted as a garage wall display, but am keeping my original H6 carburetter set up on my XK140MC…

If your concern about H6 is the Thermo Choke arrangement, as Rob R notes above, the simple fix to that ever since the 1970s when I first fully understood the weakness of the SU thermo choke design/concept was to simply fit an small/discreet under dash switch that allows you to bypass the thermo (Otter) switch, and simply engage or disengage to starting carburetter. Again, same as Rob advised. First start cold, I switch choke on, fire up car, let it run for only 10-15 seconds, then switch the choke off, maintaining just enough revs until car is warm enough to idle. Or if I am lazy/in a hurry, after switching off choke, simply drive away managing throttle as needed until fully warm. Its a lot cheaper to fit a small switch with a couple extra lengths of wire, than to source and fit properly a complete manual choke H8 set-up and then put up with all the H8 operational problems if used as a road car… But if a show car, and its all about the WOW factor - go for it… . I will keep my complete H8 set up, just twin H8s, original 2" inlet manifold, 8 longer studs, insulator blocks/gaskets, all linkages, fuel inlet pipe and complete manual choke/knob as a wall display. I also have the different Coil Bracket, but have not been able to yet source a correct Distributor, albeit if you know your Lucas Distributors you can easily mix/match and replicate…


Thanks to all for input. I have concluded that H8 conversion wouldn’t be advisable.

I have had a couple of XK’s that I have upgraded to H8’s apart from looking fantastic they do give the car more performance certainly at higher revs
example my XK120 FHC 669012 with std carbs Standing start 1/4 mile on professional drag strip with profession timing quickest I could do was 16.7sec first run out the following year on H8 was a 16.3 second run a 16.09 to scared to run it a third time it was going so well! I had a 120 km drive home.
I have 4 sets of original H8’s for my cars

Hello Terry

Interesting to hear your praise for the H8 carbs - which I am likely to pursue myself for my XK 140 C Type Head OTS.

Rare items to source as originals but New licensed repro ones are available from Moss - Bad idea?
Any chance of selling one of your 4 sets???



Hi Terry,
Very interesting to hear your 1/4 mile drag times in your 120 FHC. They are impressive and far better than I recall seeing published in various Jaguar books and period magazines. Was the engine hopped up and what transmission do you have. I witnessed a race between a 120 DHC and a 140 OTS at New England Dragway in New Hampshire a few years ago and they were running closer to 20 seconds. The slow shifting of the Moss box was partially the reason for the slow times.

I won a Drag Nite competition (bracket racing) sponsored by Jaguar Associon of New England back in 2007 with my autotbox 59 MK IX, consistently running 17.4’s. Of course in bracket racing reaction time for launching is the key and that is why I won. My final run of the nite was against a 500 HP Lingenfelter Corvette, I had a 5 second head start on him and won the race. The crowd loved it when I power braked at around 3000 RPM at the start and then let off on the brake and mashed the throttle on the second yellow light, burning rubber on the start. It was a great nite!

Best regards,
Tom Brady

The XK120 was bog stock standard with original moss box
we would have let rear tire pressures down and pumped up the front
tires Pirelli Cinturato 185 x 16
previous owner of car could only do 17.3 my first year and probably second year with the 1 3/4 was best of 16.70 but a friend had an XK120 DHC ex NZ with H8’s and was going to sell it I noted that nobody would even know what the H8’s in western Australia would be. He had just brought an SS1 ex NZ so we took the 1 3/4 inch carbs of the120FHC and swapped with H8’s from DHC and I gave him some SS parts I had. I didn’t adjust /tune or do anything based on the fact I had no idea of mechanics, but
I know how to drop the clutch and accelerate hard!

We list some 30 S/S 1/4 mile times for Jaguars in our original XK book and my
So the event would have been 1984, don’t forget in those it was a fell 1/4 mile drags now are 400metres