XJ6 SII, Triple carburetors conversion

Hello all,

I am distantly looking into a possible conversion from the current 2 HIF7s carburetors to a triple set-up for a little more power.

I wonder if 3 HIF7s would do the job? Any experience?

What type of inlet manifold would be suitable? MK 10?

Thank you.

Hi Eric,

I was very tempted to do this when we rebuilt the Series 1 engine I have in my XJ6C but went with my car’s original HS8 carbs, manifold, head etc.

Partly because we used STD NOS pistons. My engine has the -H suffix so I was thinking of using E-type /MK X/420G high compression pistons.

I have seen a couple of these with that 4.2L MKX/420G manifold and HS8 carbs in the UK, so it’s doable but not trivial to fit.

I have now almost 55.000kms behind be on the coupé, 47.000km after the rebuild, very happy with the results.


Ps. AFAIK you need the 4.2L MKX version of the manifold, the 3.8L version which is usually easier to find would give you some puzzles to be solved regarding coolant channels etc.

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I’m not 100% sure of this, but I think if you use the 420G\Mk10 manifold the front carb may be a bit high to clear the bonnet (hood). The E Type manifold mounts the carbs slightly below the level of the ports in the head, so the clearance issue does not arise. You can actually use the 3.8 manifold, the ports and studs line up, but the water gallery setup is quite different and some holes in the head will require plugging. It might be pertinent to remember, a 37 cwt, auto Mk10 does 0 - 100mph in 27.4 sec, while a 33 cwt S1 XJ6 does it in 30.4 sec, so those extra 20hp provided by the extra carb do seem to actually exist!

I’ve been running a Mk10 4.2 manifold and 3 HD8 carbs in my S1 for over 20 years now. You need a low-top HD8 on the foremost one, but apart from that it’s pretty straightforward.


What does a “low-top HD8” look like? Can you post a photo?

I’d have liked to post a photo of the current iteration of the install, but it’s parked up and inaccessible, so instead you get a photo from at least 20 years ago! This reminded me though, the header tank also needs a rework/replacement, as the standard one will whack the underside of the bonnet.

What I describe as high-top and low-top HD8’s are the length of the top bit, where the thingummajig screws into. It’s not that easy to see from this photo, but the front one is a bit lower than the rear ones.

Since this photo the engine got EDIS Megajolt ignition (17 years ago, still running with nothing required).

This is a “low top “ as fitted to E type
HD8s and also the later HS8s fitted to XJs, part number AUC 1040.
I carried out this conversion in ‘79 on a series 1 using a 4.2 MK 10 manifold.
To lower the front carb I machined the head face of the manifold , but the “low top” the more easy option. Not sure but the EType manifold might sit the carbs

to low.

Eric, looks like you’re getting some good responses. But, for even more (possibly), search the archives for postings by Jeb Boyd. He ran a Ser II with triple carbs and, as I recall, often posted about the set-up. You’ll have go go back 15-18 years in the archives.


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Those are indeed the low tops. Not clear from my photo, though I might have changed later.

I’m guessing the damper assembly is different? It looks like a brass knob rather than plastic, which might be due to having shorter threads inside the housing. So, could you convert a regular SU to a low top by finding the correct damper assembly and simply machining the top of the housing down?

Could you modify an existing XJ inlet manifold? TIG weld some new flanges equidistant along the manifold at the appropriate points and open them up, plus weld up the existing holes where the current carbs fit. Might be a cheaper and simpler solution, as there would be no issue with water plumbing. You could use a carb\ manifold gasket as a template to make up the flanges, pretty straightforward I would think. It’s then just a matter of finding a good welder.

Yes: someone did that. Look in the archives. You just shorten the damper rod and the damper tube in the piston a bit.

My Rover has the high-top HD8s.

Compared to the much simpler modifications needed to convert a MK10 manifold and header tank, this seems like a crazy amount of tricky work to accomplish pretty much the exact same thing. Keep in mind the Mk10 manifold also gives you all the linkages ready to use.

That’s true, if you have easy access to a Mk 10 manifold. Here in New Zealand there are fewer cars available for wrecking, so parts like that are harder to come by, and consequently quite expensive, hence the modification suggestion. I’m not actually sure it would be quite as much work as you think; back in the early '70’s I did a similar thing converting a 3.4 Mk1 manifold to take Dellorto carbs, courtesy of welding the carb mounting part of a Crossflow Ford manifold to the Jag. Proper manifolds in those days were unavailable then, and we Kiwis are famous for our ability to make a sows ear out of a silk purse, as this modification ultimately proved to be!

Well then Kevin !! best make a silk purse out of a silk purse and fabricate a bespoke manifold. :smiley:

I have this 4.2 MK10 manifold for sale.

Please PM me if interested.

This is the setup on my 3.4 Mk1. Provision was made to fit a 3rd carb at a later date; never happened!. The rear carb fouled the clutch master cylinder, so that was replaced by a horizontally mounted one from a Ford Anglia. The clutch was then so heavy, my girlfriend couldn’t push it to the floor. The front carb fouled the inner guard, so that was cut away. The heap ended up producing 1\4 of the power it, should, did 8mpg and, if you accelerated using more than 1\4 throttle, would spit back through the carbs so badly, that, at night, you could see flames flying out through the wheelarch.This in the midst of a fuel crisis during which petrol stations were closed fom midday Saturday until Monday morning. Needless to say, I refitted the SUs, sadly very much poorer

, but also very much wiser. In my defence, I was only 18 at the time, but since then, all my Jags have been left strictly stock.

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You sound as mad as me, I had a Hunter that I fitted twin Dellortos to, so far back that I can’t remember exactly how, but I also modified the inlet manifold of my brothers SP250 to take a Holley, tool-makings useful :+1:

Robin, if you are\were a tool maker, you are several rungs up the ladder from me. What I did was nothing short of butchery; I cringe when I think about it. That Mk1 cost me so much money it’s a wonder it didn’t scare me off Jags forever. The next one was a 3.8 Mk2 that was so rusty the accelerator pedal fell onto the ground. An auto, it had no reverse gear and only top, but I actaully preferred driving it to the Mazda Capella I also had at the time. Just had to be carefull where I parked. Then came a nice 3.8 E Type, and I was on my way, another 20 or so over the years. I actaully bought the Mk1 because I couldn’t afford a Mk2, but having now had 6 Mk2s, I would now rather have a Mk1.

This one was so rusty it was actually given to me, and it was touch and go as to whether to scrap it, but I managed to turn it into this. 3.4 auto.


Looks like it was mobile til 2011 any idea where it is now?