679526 carburetors

When placing the carburetors onto the intake manifold the kit provided 4 gaskets. A y reading I have done provides the procedure for removal and refitting. There is no mention of the number of gaskets. The question is do I use 2 gaskets per carburetor or just one between the carburetor and the manifold?

There should be isolators between the carbs and the manifold? If so thats where the 2 per carb get used.
Carb, gasket, isolator, gasket, manifold.

What does the isolated look like. All I remember is carburetors and gasket just don’t remember if it was 1 or 2 gaskets between carburetors and intake manifold. The previous owner may have left them off so the air filters would fit.

Basically the same shape as the manifold/ carb flange, 4-6mm thick and a phenolic type of material.

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The fiber insulators were added beginning with engine W6919. Your car, like mine, did not have the insulators when new. They are a later improvement which prevents vapor lock, helps with hot starting. I added them to mine, and had an immediate improvement in starting the engine when it is hot.


To add them you would also need 8 longer studs.
You can get by without them for now and add them later after you get some running experience.

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Hi Rob,
Did you have to modify the linkage system to compensate for the spacers?

I see, there was no such spacer when I removed the carburetors. I will order them and put them in in due course. Thank you for that information!

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No change to the linkage.

What about the steering column. Did the carbs foul?

Very close on a RHD car. Driver’s side engine mount should have about 1/4" spacer under it to tilt the engine a little bit. Only RHD cars had this.

The rear carb jet assembly is very close to the steering column. Some people cut a semicircle out of the steering column casing with a curved closing plate welded in to make access to te rear jet easier. To take the rear carb on and off, the jet assembly has to be removed first.

You can’t go any thicker than about 6mm on the carb spacers. LHD car owners have it easy! My car was absolutely awful to start when hot and surged really badly for about half a minute before I put the spacers in which completely cured the problem.

Thanks Chris, I tried the spacers in the past but the rear carb was almost touching. Not having had any vaporisation problems, I removed them. I’ll monitor the situation and try the engine mount spacer if necessary.

Of course chassis number 679526 indicates that there is no fouling problem there!

Rob,
I noticed on your photo that a return spring is attached to the “cranked lever” of the Rear Carburettor. Viart states that there’s only one spring for the 2 carbs which at its lower end is fitted to the screw that holds the band of the starter motor just below the Rear Carburettor.

Is that correct? So the second cranked lever on the shaft of the Front Carburettor doesn’t have a return spring? Did anyone ever introduced a second spring?

Reason I ask is that my XK 140 had 2 springs with a special bracket on both the Front and Rear Carburettors to hold the lower end of each spring. My experience is that a second spring was absolutely required in order to obtain that both carbs close the throttle disc in a reliable way, especially when after many miles wear of the various control shafts and levers starts playing a role.

Bob K.

There should only be one cranked lever (at least on the XK120) which is correct as in Rob’s picture. The more spring pressure, the greater the tendency to wear the spindle bushes, I would say. The standard setup means there would only be a bit of spring pressure on the rear spindle bush of the rear carb, and none at all on the front one. Just enough spring pressure to reliably close the butterflies should be all that’s necessary. If wear has developed on the spindles, then they should be re-bushed, as air leaks there will cause an erratic and unreliable idle, and extra springs will certainly not help with that condition.

Let me preface my response by saying I am not an expert on SCCA racing, but I recall reading many years ago that at some point SCCA began requiring a return spring on each carb, obviously for safety in case of linkage failure during a race. Possibly other race sponsors followed suit. It is possible your car was set up that way for that reason.
Here is the response that MG came up with, rotary springs on each shaft. At least they would not add to the shaft wear.

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That does seem a reasonable explanation for two springs, Rob. The co-axial springs fitted to some other makes does seem like a better solution.

Chris,

The problem is that there are two cranked levers fitted: according the XK 120 SPC both Front and Rear carb should have lever Jaguar 1635 or AUC 3120. This lever is now also known as AUE 206 or AUE 230 (slight difference in length).
Also many photos of original XK 120 engines show the two levers. But only one has a spring Jaguar 310.
I’m trying to understand what the idea of the Jaguar engineers behind this all was: if one spring is sufficient, then why not fit only one lever? Of course your remarks regarding the “state” of the mechanical components is valid, but that will gradually change unfortunately. Also Rob’s remarks on the safety aspects should be kept in mind. And finally: why did Jaguar introduce a second spring on the XK 140 if everything was fine on the XK 120 with just one spring?

Bob K.

Addendum.
Just saw that Jaguar introduced (Autumn 1952) a second return spring on the Mk VII after having produced 6000 LHD cars with only one spring.

I have checked the SPC and you are indeed correct, Bob. I certainly can’t explain it! I’ve checked in both volumes of Urs Scmid and all the pictures seem to show only one lever. There is one picture that possibly shows the securing nut on the end of a lever behind the front carb, but no view of the other end of the lever, if indeed that’s what it is. Perhaps it’s just an anomaly in the SPC which didn’t make it to the real world?

I’ve no experience of XK140 so will take your word for it that they used two levers and springs.

Interesting tidbit of info on the Mk VII, too!

Chris

This from the Mark VII SPC shows three levers, items 11, 11 and 68, and initially all three are the same part number 1635 (3120/1).
image
Apparently later on one of the levers was different, and they had two return springs.
image
And they added the fiber insulators to Mark VII about the same time as XK120.
image

I found this pic of a Mark VII engine with what looks like two spring brackets.

No time to check now but it was a safety/regulatory issue that took effect from later XK120.
Earlier XK120 had one spring - and we picture that in the JCNA XK120 Judges Guide…
But later XK120, and thereafter, so all XK140/150 etc the safety regulations deemed each carburetter that was not solidly connected to its twin, needed a return spring for both carburetters. And with XK twin SU set-up, the flexible coupling between front and rear carbs was not sufficient, so both carburetters needed their own individual return spring, positively located on the throttle shaft…

JCNA allows two springs om all XK120s, as its recognised as an allowable ‘safety’ modification, if not original for earlier cars. So your choice. One spring is enough if you make sure all your linkages are secure, but two springs - and associated brackets - were deemed an ‘improvement’

I keep saying - XK140 - is a much improved car in all respects to an XK120 :grinning:
So nothing wrong with upgrading your XK120 to XK140 specs…

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Hi Bob:

My car, a July, 1953 build DHC, has the two levers, but only the rear one had the return spring attached to the starter strap. Some time ago, assuming that the spring on the front carb had gone missing, I fitted one attaching it to the oil pressure gauge pipe coming off the filter assembly. This made depressing the accelerator a bit too severe so I reverted to using just the one on the rear carb after experimenting with lengths/tensions. Rob’s theory is interesting, although I wonder why some Mark VIIs had two springs fitted?

Chris.