Triple HS8 on 420G manifold, with manual chokes

(tony) #1

I am looking at installing a 420G manifold with triple HS8 back into my 420G

At present it has a 9:1 XJ motor, with dual HS8, and manual choke setup.

I never did like the original triple carb setup, mainly due to the ASC, that came with the Saloon HD8 equipped cars, cause petrol fumes

I have 4 x HS8 carbs, and a spare 420G manifold, so fitted some carbs to check whether the fuel bowls, and throttle linkage will work out, and any other fitment issues.

The good news is it appears the orientation shown will fit without any apparent clearance issues

It looks clean, no Jaguar part will be injured. I think I can do it with parts on hand

I will have to obtain a length of 1/4" rod, to shorten the linkages, instead of cutting the originals

(Phil.Dobson) #2

what choke set up will you use?

(tony) #3

The choke setup will be the one that is in my car now, pictured.

It has Burlen Parts, and works very well

Presently controlled by a single locking cable

I will most likely fit or adapt a dual choke cable, used on Stromberg equipped vehicle

One choke will operate the rear pair, as now (front cable)

the plan is the front carb will be operated independantly by the 2nd cable

the carbs are connected with 8mm rods, I will cut some brass rod, and reuse the levers

the rear cable in the pic is my throttle cable conversion, I made it a bit too long,
this will probably be modded for dual throttle cable

(Robert Wilkinson) #4

Very nice conversion, Tony. I dislike the hisser setup too. In addition to petrol fumes, mine occasionally coughs up a few drops of petrol onto the wing, where it burns neatly for a few seconds. So far only parked with the bonnet open, but I worry.

(tony) #5

Yes, I saw a car nearly burn in a workshop due to that, would have without a fire extinguisher.
Its vital the small triangular plate over the fuel well drillings is in place…is it on yours?

fuel vapor is just wafting out those drillings at all times, that source of smell can never be eliminated

probably the most important thing for me is most of my trips are very short, and the ASC is an on/off device, and to start and run initially must be rich, which is no good after about 30secs, causes a lot of issues on a worn motor

With the manual choke, it beats rich after 30secs, I half it before driving off
The full manual choke has 3 adjustments and cold idle can be set very accurately with respect to RPM & Jet depth

next step, might set it up on an old block I have, to double check a few clearance issues, it appears I dont have to move anything, and have ordered a new heater outlet spigot, the old one is rusty…it should just fit back up against the S2 throttle plates

Robert, maybe you know?

might not be easy to get the old one out

8mm brass rod should be here next week, so I shall assemble it and fit on the old engine

had to hurry up and put it all away, as I was working in the kitchen & front deck while the Mrs was away working

(Robert Wilkinson) #6

Not sure I understand, Tony. S2 throttle plates?

The heater outlet spigot (3/8 BSPP) was the subject of a recent lengthy E-type thread. It appears that wrench purchase onto the hex portion is more difficult on that manifold than it is on the 420G version. Anyway, the lister finally got it out, and replaced it with a brass one.

(tony) #7

I believe you have a 420G manifold fitted to your S1? XJ, with XJ cable throttle

The spigot we are talking about is the same one that was subject to the E-type thread

It will may close, but it appears the 420G spigot (with rubber hose) will not interfere with the S1/S2 throttle linkage plates that are bolted to the cylinder head at the back?

If there is interference, I would need a right angle replacement

The S2 XJ manifold has a right angle heater spigot…I do not know its thread size.

The spigot is shot, and must be removed in any case

(tony) #8

supplementary question.

I seem to remember a lister form Australlia or NZ,

Neville ?, has a tri-carb setup, ?
but only choked 2 of 3 carbs

This would be very easy to setup

any opinions or knowledge?

The choke is only used for about 2-3mins on a cold start

(Robert Wilkinson) #9

Just checked–the spigot is outboard and slightly above the plate that is attached tot the upper and lower rear manifold studs. On my car there is generous clearance–more than an inch.

Yes, I recall the discussion of manual chokes, and IIRC two seemed to work fine. I would even try just one if I had the bits to convert my centre HD8. Note that with the hisser, all three throttle plates are closed, and mixture is fed into the manifold at only two locations–again suggesting that two choked HS8s should be adequate.

Forgot to respond to your earlier post. Yes, I have the little triangular sheet metal piece on my ASC (hisser). There was an even bigger, roughly rectangular, sheet metal piece supplied by the factory for some Mark Xs. A local club member here has one, and documented for me its factory origin. I made one, but removed it because it fouled my finger access to the nuts I use to secure my air box.

(tony) #10

I got a bit more done today, the old heater outlet tap is out,
courtesy of my Milwaukee impact driver, thread is all good, new one on its way

The first fitment issue has arisen , that being the 420G manifold protrudes ~35mm further out
from the head then the S2 XJ manifold

This means the S2 throttle cable assembly cannot line up the throttle actuating rod to the carb, without also being moved a corresponding distance away from the block

@ Robert, did you encounter this, and what was your solution ?

The problem is illustrated by the pic below, altho the gap shown is even wider, I can fit a block, and extend the brace so that it will bolt to the head, but protrude outward

I must also cut the main throttle actuating rod in the mechanism, to make it shorter, to account for the rear carb being further rearward, no way around that I can see


(Andrew Waugh) #11

A couple of years before my 420g came into my possession it suffered an engine fire which started at the hisser. My mother had been for the MOT, and the fire started about 5 min away from the testing station. We were pretty sure that one of the mechanics had knocked the solenoid loose on what would have been to him an unfamiliar engine.

Years later I was on a rally in a Mark 2, and on a speed section I hit a bump hard enough that it dislodged the solenoid, which was then dangling down in the now fuel soaked area under the carbs.

I now always fit a tyrap around the stirrup holding the solenoid, locating it down near the two holes in the stirrup, so that if the cap cracks or the screw loosens the dangerous bits are at least where they won’t do any damage.

1 Like
(Robert Laughton) #12

During my morning forum rounds, I saw this:


(Robert Wilkinson) #13

I am not at all pleased with what I did, Tony. I tied the throttle shafts of all three carbs together (just like two are tied together in dual Strangleberg setups). I then connected that to the (extended) factory XJ Series 1 shaft with a link. I have problems with maintaining a smooth idle, and the throttle butterflies don’t fully open.

But I don’t understand your problem. You are fitting a 420G manifold to a 420G. It would seem that, using factory bits, you could connect the 3/8 inch shaft, that runs above the manifold in a couple of lugs cast into it. I removed that shaft, which was a big mistake. Then, all you have to do is link down from each of the three levers on that shaft to the individual throttles of the HS8s. This linkage would presumably differ from what it was using HD8s, but it seems that modifying it would be the easiest way to connect your carbies.

I suspect I may be missing something? If you’re just trying to retain the linkage you used with the dual carbs, I can see the convenience but IMO switching back to the original 420G bits would be well worth it. I’m still working on mine a decade after fitting them.

(tony) #14

I will post more pics once I have a better mock setup, its hard to describe, sounds like you pursued a different solution…do you still have the throttle cable & plate mechanism bolted to the back of the head?

The issue is the throttle plate assembly pictured (held) above, you can see a gap between ~350 to 400mm on the ruler, and that is how far away from the head the assembly has to be to line up the throttle actuating shaft

that is because the 420G manifold extends 35mm further outward than an XJ mani

the throttle assembly bolts to the back 2 bolts on the inlet manifold, and 2 small bolts on the rear of the head…on an XJ

By making a 35mm thick block, and a lever extension, I can still mount it like that

I do not wish to return to the 420G throttle mechanism, It did not work at all well with the present dual HS8, hence I converted to a cable operation, based on XJ.

The present throttle cable system works well, better than the original triple HD8 setup

after mucking around with the cables for ages, I recently came across an E-type guy that has done similar, but with dual throttle cables…I will add a link later

will also add pics of the choke parts, when I take it all apart (a while off yet), its has quite a few extra parts

(Robert Wilkinson) #15

Yes I do, Tony. It doesn’t line up either. @RustfreeMike recently did this conversion recently. He did a bit of welding. I’ve seen a photo but I just looked through all or our email correspondence and can’t find it. Hopefully he will see this but if not you should inquire via a PM.

I didn’t know that the original throttle linkage was bad. Sounds from your description that it wasn’t a cable system.

(tony) #16

The 420G original throttle system is not cable, but instead has quite a few rods and pivot points, it isnt bad as such, just far more complicated than a cable system, and (I found) it dfficult to adapt to the XJ engine

I had to design a system that was specific to a 420G auto, with XJ engine & carbs

Thx, I will PM rustfreemike…know another fellow did it, I will ring him

(tony) #17

picture tell a thousand words

I made 2 small brackets, circled in red, to accommodate the XJ throttle cable assembly
needing to be 40mm further out from the head

(XJ manifold is 90mm, 420G 130mm)

the throttle rod, in yellow, needs to be trimmed by the distance highlight in blue, to accommodate moving the carb backwards (with respect to a twin-carb)

I saw @RustfreeMike setup, he uses the 420G throttle parts

In my case they are all removed.

My brackets are just fabbed from scrap, study items will be made. Looks like everything fits

(Robert Wilkinson) #18

Magnificent! I didn’t know Mike had used 420G parts; thought he had extended like you did. I wish I had my original linkage parts–they seem to have been scattered about. If I did, I would try to duplicate what you have done.

I see you are using plain 1/4 inch insulators (E-type style) instead of the bulky spring loaded O-rings used on the Mark X/420G (but you still haven’t gotten around to replacing the studs with plain ones). I assume that the shock-mounted insulators were intended to isolate the entire assembly with its heavy air box. I used plain insulators, plus the air box, and so far have gotten away with it.

(tony) #19

the insulators at present are the spring-loaded items, as are the studs.

I will swap & use the shorter XJ studs, as the longer ones make life hell to get a carb off

I have a choice of old or newer style insulators.

My throttle rods & other items should turn up in the next couple of days

(Robert Wilkinson) #20

Not sure I follow you, Tony. If you use the shorter studs then you won’t be able to use the springs? If so no reason to keep the o-rings and metal plates. The phenolic insulators (as on E-types) are IMHO the way to go. Unless you want to have the vibration isolation feature…but that would require the long studs, IIRC.