Hi there!

As I can't really be arsed to fiddle around with the injection system on the V12, I was wondering: were there any "common" carb conversions in XJ-S models?

Not saying conversion like V12 out, V8 in. But in the sense of using say 12 single carbs or 6 Weber doubles and co?

Reason being: there is a way here for me to legally get a historic plate even though the engine is "modded". That requires knowing what was a common conversion. And if I then do that conversion, the car will get the tax bonus and valuation gains I intend it to get...

Any ideas?

 

Daim,

Not common for your vintage of course, but some folks put XJ / E Zeniths on, next to webers of course. I'm not entirely sure it's a simple manifold swap due to later head design, but it could be I suppose.

Webers down the center are the bees knees, but you will likely have a clearance issue to the bonnet, in which case, side slingers are your huckleberry.

I suspect you could get four Zeniths and manifolds / other kit for sub $1k US. Webers, not so much..

Jeff

 

I was expecting to have to lay them or better fit them over the valve covers due to the lack of bonnet/hood height - unless of course a later AJ16 bonnet/hood was fitted (should allow for a little bit more clearance if I am correct). Anyhow, I'd love 6 Weber twins along the middle of the V... That would be soooo nice

 

Although I'm sure its not what you're looking for, there was a twin 4-barrel conversion marketed for the V12 at one time.

https://www.jaguarforums.com/forum/x...intakes-67766/

 

If you think the EFI is too much work to get running well, I can definitely say that multiple carbs aren't for you!
If I HAD to do any carb conversion to a Jag V12 and have any hope of driveability, reliability and ease of maintenence, it would be the original Jaguar setup, followed closely by two small 4 barrel vacuum secondary carbs on a custom manifold.
The question is- how much work are you prepared to put into getting it running? Because I can assure you, a custom manifold will easily run thousands of dollars if you pay someone to fabricate it and it will take at least 40 hours to get it tuned across the whole driving range.

 

Typhoon 100% right, Daim. The HE engine is not that suitable for carbs and will give you catastrophic fuel consumption on Webers, if you ever get them even roughly tuned.


Your best option if you want carbs is to change the heads/engine for the pre HE saloon or E Type factory carbed setup. But can you explain your aversion to the EFI system? Once the loom bits in the engine bay are sorted (if they need it) it's a pretty reliable system I have found.


Greg

 

I'll take carburation any day, the right kind. On our xj-s's 2 450cfm Holley's would work great. The Problem is hood clearance, as already mentioned. Side draft Webers look great but we all know the cost (yea EFI & parts are downright cheap......) I don't know about the driveabity/ tuning. Never had any experience with them. I see them at car cruises all the time. Anyway, they couldn't have to many more problems than our EFI from what I've been reading on the forums. Sensors, SAV, AAV, ECU. idle problems, etc. Its really all irreverent, carburation on a xjs is dreaming.
Lawrence

 

Which engine type do you have in your carb'd XJS, Lawrence?


The factory V12 in E type form and on the series I and II saloons had factory carburation. This is the pre HE engine, mind. This setup will fit under the bonnet of an XJS.

I believe The HE engine is not really suitable for carbs, as the high compression and poor combustion chamber shape will always give combustion problems and cylinder air/fuel mix problems at several points over the rev range, even if by a miracle you manage to get decent running at some revs. Then add in detonation at high revs that cannot easily be offset by the ECU telling the system to go full rich, catastrophic fuel consumption (maybe 8 on webers) and it looks guastly.

So it is pre HE engine or forget it, I believe. I can always be wrong, of course but undoubtedly there would be a mountain of problems to solve carbing an HE.

Greg

 

A guy in my local club has strombergs on his XJS. Swapped them from a XKE. He likes them, but says it gets about 8 miles to the gallon, which is about what my 71XKE got. EFI really isn't that difficult, and really is FAR better for drivability. The best thing about carbs is wide open fuel delivery

 

In reality carburating a xjs v12 just isn't practical in regard to hood clearance, streetable affordable carbs, etc,etc. So if I was that set on scraping the xjs EFI I would seriously consider a V8 swap using a small Holley (ie 700cfm) & MSD ign. system. HP, torque, cheap speed parts are limitless . No computer, sensors, AAV's, SAV's or idle problems to ruin your day, but the emission people might.
Lawrence

 

I have not attempted a carb conversion, I have been fighting to get the original FI system working properly, and I am getting very close.
If I wanted to go with carbs, I would try putting a single SU or Zenith on each manifold where the throttles bolt on. But I bet you would have as much time and effort into that as you would into getting the FI working.

 

Agreed and the stock early Bosch/ Lucas systems are ridiculously simple to repair and diagnose. I'd much rather sort it out than balancing multiple carburettor throats, synchronizing throttle blade openings, playing around with emulsion tubes/ power valves/ transition orifices/ automatic chokes/ secondary throttle opening etc to try and get something resembling half decent driveability out of carbs.
Even with an exact starting point for a set of carbs to just "bolt on" there will be countless hours of tuning.

 

I don't know what type of carburetor has all those problems (maybe Weber's, never worked on them) but what is more simple & more forgiving than a 600 to 800 cfm Holley, double pumper or vacuum sec's. So the way I understand the reading, Jaguar EFI is much simpler to diagnose, less complicated & easier to work on than a Holley or AFB. Setting Jag EFI Idle? Better read the Forum's for that.
Lawrence

 

SU's and Zenith's are just as simple and stay in tune for virtually ever / don't require much fiddle unless you need to get through a smoke house or the like (in which case - God help you ;-).

Webers really aren't brain surgery either, I've gone MANY years in between messing with settings.

Just like everything else, things wear out / bushings need replacement and our lovely crap gas speeds up death.

Based on the long intake tract, unless you put Weber's down the center (to get rid of the same) I suspect performace would suffer quite noticeably with either four Zenith's or some retro-fit of down draft on the stock intakes. it works well on the XJ and S3 E, but you're hooked to a four speed, different ratios and a different torque curve due to head design.

+1 more for staying with the stock EFI setup..

Jeff

 

Any fool can bolt a carb onto an engine, set idle mixture and proclaim it's tuned. It is not.
The amount of carb swaps I've seen that have performed woefully is testament to this. I have plenty of experience modifying and tuning carburettors of all types and quantities, so you can take my statements as fact, not the forum regurgitations of some internet mechanic.

 

I looked into doing the carb swap. I prefer carbs over injection. Carbs ste simple and reliable.
Found it was grossly expensive for what I wanted. I looked in to the dual quad set up. The intakes were $1000 for the pair. $800 for 2 small Holley carbs(500cfm) $200+ for linkage. More $$$ for custome air cleaner. Then theres the hood to deal with coz they aint gonna fit under the hood. For $2500 you can easily restore your fuel injection.
It would really nice though.
Looked a 4 carb side draft as well. Didn't see the point in that. Wasn't going to be a performance upgrade.
My 2 cents worth

 

As I started this topic and have had little time lately, I think I should answer why I intended on changing to carbs...

1. I always like the look of some Weber twin carbs... I was thinking a set of 6 twin carbs as side drafters...
2. People "worry" others about engine fires due to brittle hosing and wiring. My hoses have been replaced but before I rebuild my injector harness, I considered the "mechanical" carbs instead
3. No Holley or Edelbrock for me! Either Weber or some British carbs. Nothing else.
4. I am aware of the high engine bay temperatures but considering that my '89 V12 sucks in though the warm radiator and not though any cold air drafting, it wouldn't be any different.

So, it was first of all for the sake of keeping hoses and wires out of the V and secondly to have something pretty with a nice crackle and misfire through the exhaust when driving (no cats so no Lambda sh*te needed ).

 

Cams are different between fuel injection and carbs. When using carb cams on fuel injection, too rich. Carbs on fuel injection cam can be too lean as fuel injection is a computer managed system. Carbs are a manual managed system.

 

Check with M90power on forum. He has a set of stromburg carbs and manifolds. I spoke with him last year. He may still have them. Heres a photo he sent me.

 

These are from the early flathead V12 versions used in the E type and the Series 1 and 2 saloons. They require, as someone explained above, different cams and even more importantly, heads, to work properly. In my view carbs are not a bolt on proposition for the HE engine with its high compression ratio and odd combustion chamber shape.


I quite understand the attraction of carbs, although our V12 HE engines have far more trouble with their ignition systems than their fuel injection which is quite separate. It is just that I do not believe a carbed HE engine will run at all well, if even driveable at all, and Daim will end up with a dreadful car he will not want to drive that uses huge amounts of fuel.


Greg