Triple SU Carb Purchase for my XJC

Hello Everyone,

I’m trying to figure if this purchase would fit my XJC. From what I can tell they are HD8’s with a Thermo Carb. I understand I may have to get a different manifold to make them fit.

Those look to be from a Mark X 3.8. The hisser (thermocarb) is damaged/missing bits.

Yes it will fit, but with some fettling. You need to fabricate linkage. You need to block two water passages in the 4.2 head or else substitute a 4.2 manifold. You need to cut down the necks on the suction chambers. And, in my case I needed to angle the manifold-to-head surface a bit to get the front dashpot cap to clear the bonnet. This last step seems to subject to luck (you can also rotate the engine CCW on its mounts as much as possible. IMO, depends on the price–the hisser bits aren’t cheap.

Should fit fine. They are 3.8MkX. Don’t over-pay as they don’t fit an E-type. You will need to block two water passages with epoxied freeze plugs.

I can purchase them for $600.

Can I modify the the thermocarb to a straight carb?

Why? The ‘hisser’ starting carb works and converting to manual choke puts you in E-Type price band…

Perhaps use a manual switch for the electric starter carb if you don’t trust the reverse-acting Otter thermoswitch (on when cold, off when warm).

Disclaimer** I’m new to the Jaguar Community. I’ve been in love with their classic cars from when I was young. I just bought a 1975 XJC 2 weeks ago and planning on a restomod. I apologize in advance if I ask a rookie question. I’m not as seasoned as some here which is why I’m inquiring into your knowledge.

The reason I was wondering if it can be modified was because Robert mentioned that the hisser was damaged and missing parts. *Note: I don’t even know what a hisser was until this morning. I’ve personally never seen one before. :roll_eyes: Also, I couldn’t find any tune up kits for it. I even went on SU’s website to see what they had for it couldn’t really find parts. I emailed them already. If they rebuilt it is $380 plus shipping to and back across the pond probably not the most cost-effective way.

PeterCrespin… $600 too much?

Bradley,

I can’t see all of the starting carburetor in your photo. It’s clear that one of the electrical contacts to the solenoid is missing, as is the bakelite cap and metal retainer. All that might well be repairable. There is tubing from it to the intake manifold, which I can’t see in order to confirm that it is there. Regarding price, I think $600 is reasonable. I paid more than that about 20 years ago. I assume you are in the US. If so, note that the triple SUs won’t give you a huge improvement over twin Stranglebergs (Crespin’s term, now firmly entrenched in the J-L argot; yes we are a weird but friendly bunch). But they provide “eye candy.” There were twin SU setups for XJs as well…HD8s in Europe, and HIF7s in Canada IIRC. And of course you can fit EFI, which began a few years after your coupe was built.

the manifold is fine, even uses takes the same gasket

As the XJ has a cable throttle, and the MKX uses rods, you will have to fab the throttle mechanism

post close up pics of the solenoid, you could probably purchase the few bits you need via the “classifieds” in this forum

I think they may foul the 2 x engine bay stiffener rods on one side

Bradley,

at 600 US for triple SU carbs + the inlet most people around would lick their fingers - no matter whether the parts are needed, but because these things have becom rare and expensive. In the worst case you would sell them at twice the price … well, at least if the carbs are roughly functional etc.

That being said, you have to know in which class you’re going to fight: you neither mention what your starting point is (I think XJC ended production before US SII sedans received fuel injection, so it might be twin Strombergs, but you should tell us) nor what your goal is - restomod covers so much. If you’re out for a mean looking Cobra killer you’d need some serious vee-8 power, if you want an evocation of the Broadspeed XJC racers you’d need a nice Jaguar 12 cylinder, if you just want to pep up the original the easiest way would be a later fuel injected inline 6 or - my favourite - the original twin SU HS8 setup. In good shape they turn up occasionally in about the same price range as “your” triple set up.

The triple setup may look nice, but might be a gas to get tuned and stay tuned: sync 1 and 2, sync 2 and 3, sync 3 and 1, start over …

Good luck with your new car - it is a classic and it is a beauty! While restomods sometimes give a new lease of life to otherwise moribund leftovers of cars, you might question yourself about the “mod”, if a simple “resto” will get you the original - not withstanding simple and helpful unobtrusive upgrades - typically with a great deal more market value.

Best

Jochen

75 XJ6L 4.2 auto (UK spec)

My goal is to make performance/safety/comfort upgrades utilizing Jaguar parts throughout the years.

I’m not going to swap out the beautiful XK motor for an SBC. I’m not a fan of that. As soon as you change certain items I believe you lose the appeal of the car.

It funny that you mentioned the V12. Local there was an individual that was selling a 1974 XJ12 that I could get for $600 (if still available). Would that be a better bet instead of the carbs?

1 Like

You bet it is! The 12 cylinder changes the character of the car completely. It is 20 years younger construction-wise, pumps out roughly twice as much power in SII setup and replaces the long stroke low-revving pace of the six with a turbine-like whirring up to 6000 RPM. If set up correctly and treated half way decent, in particular not over-heated, it is a much more robust engine with standard mileage of 300.000 kilometers before wear and tear start to show.

Of course, this engine changes the challenges ahead of you a lot: A carbed XK engine is straightforward engine building: you look at it and see most everything you expect about an engine. The XK engine also is something over which you may get acquainted with old cars. It is DIYable in many respects along the periphery, and simple jobs like replacing a coil or an alternator, changing oil and filter, checking plugs, swapping out a shot temperature sender are never intimidating.

By contrast, a fuel injected (it should be; the carbed versions are not desireable IMHO) V12 + air condition in the same compartment is not for the faint hearted. Almost every time it seems you have to take apart half of the engine bay just to get access. A former Leyland mechanic once told me that one of his first jobs as an apprentice was changing all the belts on a 12 cylinder. It took him the best part of one day … I know why I went for the six:-)

But clearly, if the engine is good and you have a reliable person to do the swap (if it is a fuel injected car and you have a carbed car you also need return fuel lines to the tanks and different tanks) it would make a great upgrade. We have two listers who did that and are very happy about it. Truth be told - I’d never do that without a complete donor car both as a 3D pic of where-to and to simply take all the fixings, different follow ups (from speedo drive, speedo to different fuel pump routings etc.).

Otherwise, keep searching for “Jaguar SU HS8 carbs” - they also used SU HIF7 carbs on later SII models, but I’d prefer the HS carbs.

Good luck and post some pics of the car!

Best

Jochen

75 XJ6L 4.2 auto (UK spec)

1 Like

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It’s not a bet, Bradley - performance wise it is a certainty, as Jochen says…:slight_smile:

Transfer a carbed V12 to carbed xk car is fairly straight forward, but converting any car from carbs to EFI ain’t - a lot of extra (Jaguar) parts and work are required. It would be a labor of love…

Frank
xj6 85 Sov Europe (UK/NZ)
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Don’t forget you’ll need the transmission for the V12 as well. And EFI it, as if it’s an issue syncing 3 carbs, it’s even worse with 4…

The better way forward is the fuel-injected 6-cylinder AJ6 engine from the XJ40 (1987-1993), in 3.6 or 4.0 litre form, with the more modern transmission.

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Which requires even more electronics, Nick - and spares sources…:slight_smile:

Actually, the three carb set up is arguably the most powerful version of the xk - except going with three double-barreled ones. The prime concern going with EFI was fuel economy - not more power…

Frank
xj6 85 Sov Europe (UK/NZ)
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Putting it on an effort vs. effect scale, I’d always end up with a removal of power-eating emissions stuff like the air pump (if your engine is already equipped with one) complemented by a retro-fit of the original 2" carbs - original, economical, easy to maintain and with sensible step-up in oomph … in good shape and on a SWB car they make for a quite lively drive. Maybe you have a short ratio diff and this will be even more pronounced.

Good luck

Jochen

75 XJ6L 4.2 auto (UK spec)

Not a huge amount, Frank, the AJ6 conversion has been done quite a few times now. Granted, it helps if the car is already EFI, so the fuel plumbing is in place, but it’s surprising how easily the engine slots into place. Tri-carbs don’t make much of a difference in real terms, the twin HD8 SU’s already provide more fuel than the engine can swallow. When I converted I found there might be a bit better response, but not a massive amount. The AJ6 engine on EFI is rated around 240 DIN HP (as I recall), which is more than the 265 SAE HP that the XK offered in it’s most highly rated form.

The very best upgrade on my car? The EDIS/Megajolt ignition. Now that DID make a noticeable difference.