679526 carburetors

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).
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Apparently later on one of the levers was different, and they had two return springs.
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And they added the fiber insulators to Mark VII about the same time as XK120.
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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.

First of all, my apologies to Maddy for practically “stealing” her thread about the gaskets to be used for the later carburettors and changing it into the subject of return spring(s) on XK 120 carburettors in general.

The many contributions however have lead to more understanding of what (may have) happened.
Although there are two “cranked” levers (Jaguar 1635 or AUC 3120) there’s only one Return Spring (Jaguar 310) attached (the lever of the Rear Carburettor).

We also know that the Mark VII changed over from one spring to two springs in the Autumn of 1952.
And at the start of production of the XK 140 in October 1954, the standard engine was more or less identical to the late XK 120 versions but now had a return spring for each carburettor. Jaguar changed the construction for attaching this Return Spring (Jaguar 310 and identical to the XK 120 version) by adding a separate Bracket C. 8015 positioned under the lower LH nut of each carburettors. As Rob mentioned already, these are the same Brackets that were added to the Mark VII in the autumn of 1952!!!

Roger P. made the remark that “later XK120, and thereafter, so all XK140/150 etc…needed a return spring for both carburetters.” Roger: do you also know whether Bracket C.8015 was used on these later XK 120s? It seems a logical assumption, but does anyone ever seen a picture of a late XK 120 engine with these Brackets?

Finally, it seems (still to be measured) that the length of Bracket C.8015 is shorter than the distance from the cranked lever to the screw of the strap of the starter motor. Furthermore, the bracket has two holes allowing to choose where to position the lower attachment of the spring. Both changes would compensate for the increased spring force by doubling the number of springs as Chris mentioned earlier.

Bob K.

Bob,

Apology accepted but not necessary. The spring information is valuable in that I have to yet put my carburetors on and will need the return spring (or 2).

I will have more carburetor questions when the time comes and will continue the dialogue here.

Maddy

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Nope. You need to appreciate when an Australian was co-authoring a JCNA Judges Guide, I had to accept USA local knowledge of things I was familiar with based on Australian local knowledge. My immediate position was that XK140 introduced the two spring arrangement, but local Australian regulations required XK120 to add a second spring for safety reasons - and retrospectively, an XK120 was not considered roadworthy thus registerable unless modified to have a second spring. All the XK120s I had seen with two springs looked to me to be local modifications/non-SU parts etc, and I had not seen anything that looked factory original; but I suggested if I was going to modify an XK120 myself, I would replicate the XK140 arrangement, thus with a C.8015 bracket. But I was assured that ‘late US XK120s’ did have two springs, with the peer review consensus (all US local knowledge) that further research/evidence was unnecessary as it was an ‘acceptable safety related’ feature that was not therefore deductable. In Australia, we also allow two spring returns on XK120 as its a State legal/registration requirement, and we dont comment/care how it is exactly done, but expect with all appropriate period SU parts, and not some local speed-shop modern brackets/arrangement. So when Judging (in Australia) I would be happy if someone fitted an XK140 two return spring arrangement, or if they retained the quite different single XK120 spring arrangement and added a second spring return using period/correct SU lever/spring/brackettry…

But maybe someone has an extremely original late 1954 XK120, and can comment on whether one or two springs - I have photos of original XK120s as late as 5 July 1954 that still have their original single spring from a lever at rear of rear SU, extending down to the setscrew the holds the starter-motor cover in place… . See pic below… shows the rear-lever (original) and nothing on front throttle-shaft/SU

This seems to be one of those questions where we don’t have a definitive answer from the factory literature, so we have to stumble around and throw ideas on the table to see which one will fly.
The XK140 SPC at least has a picture.
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Item 80 Bracket Anchoring Return Springs is part C.8015, also used on Mark VII, and it is a reasonable assumption that part numbers were assigned consecutively as they were designed in the engineering office, which is a high enough number to suggest that it was designed and the number assigned later in the XK120 era.
The wording in the Mark VII SPC might lead one to suspect that the two springs requirement did not at first apply to all markets.
However, Service Bulletin 128 June 1953 says they were putting two springs on Mark VII for more accurate slow running settings (and anti-creep setting on automatic transmission cars). Brackets for the springs were attached to the lower carburettor flange studs.
So I think we have a date for the bracket, May or June '53.
I did not find any other SBs mentioning throttle return springs.
Those of us with 120s with a single spring know that it is tricky to get the two carbs balanced together. Two springs would make it easier.
No 120 had an automatic of course, but perhaps they began putting them on all 140 engines because some of them did have automatic.
Just more ideas on the table.

An update on the use of 1 or 2 return spring levers on the XK 120/140.

Decided to opt for the 2 spring solution and thought that I could simply use two levers Jaguar C.5606 (or SU AUC 3120/1). But I soon learned that this Lever is too long and will touch the Thermostat Unit (AED). See photo below.

Both the Mk VII and XK 140 use this shorter Lever for the Return Spring on the Front carburettor (Jaguar C.6930 and SU AUC 1500/1, later replaced by AUE 180). This lever is straight (not cranked) and shorter than the (cranked) Lever for Return Spring of the Rear carburettor (Jaguar C.5606 or SU AUC 3120/1). The longer one AUC.3120/1 is about 1.5” between the holes and the shorter one AUC 1500/1 about 1”.

Note that Viart (XK 140 Explored) made an error and changed Jaguar part number C.6930 in an SU number AUC.6930. Idem for the longer version AUC.5606 which should read C.5606. Couldn’t find these SU numbers…

Bob K.

Bob:

I took at look at the return spring lever on my front carb and, looking at it from the inlet side (opposite to your photo), it is cranked over to the right towards the idle adjusting screw and clears the starting carb by about 1/4". It is difficult to get a measure, but using a pair of dividers I got 1 1/4" between the holes, the same as the rear carb lever. If interested I will try and get a photo.

Chris.
677577

Chris,

So you say that the “longer, cranked” Return Spring Lever does fit if it’s turned 180 degree. I will give it a try later today. Nevertheless Jaguar opted for a shorter Lever.

Bob K.

Chris,

Had a try but the longer Lever doesn’t fit no matter how you turn it. You refer to 1 1/4" between the holes whereas the original longer version (AUC 3120/1) measures 1 1/2" and the Jaguar listed shorter version AUC 1500/1 measures 1". So I guess yours is yet another version (see list below) like AUE 191.

Bob K.

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