Black SU Carb Piston Chamber

I imagine this will light a fire under one or two boilers. :grin:

Here is a piston chamber from a Mark V that I acquired a couple of weeks ago with a stash of Mark V parts.


Unknown source car or history and no other carb parts in the stash.
It had an FW needle in it which is correct for Mark V.
I wonder how many of them had black pots?

I once saw another Mark V with black suction chambers, a very early one.
It was 627025, engine T5052, sitting outside since 1960,
overgrown with trees and sunk into the ground up to the frame.
That was in 1997.
I went back a couple of years ago and it was gone and the house was abandoned.

Rolls Royce/Bentley hada thing for enameling the suction chamber tops in Black, and other parts. It was also done for Motor Shows abit and I have seen pics of MK V Motor Show engines with this done. I also did a little bit of work on a Daimler DB drop head that had been the Melb Motor Show car and it also had this done.
I am a bit inspired to to do the saloon tops this way, just to be different.
I recall an SU carb rebuilder in USA advertising carbs done like this

Of course if youwant the car to go faster they should be enameled in re
In some ways better than the concours would be’s who polish the tops to within an inch of their lives and distort them so the piston doean’t move properly in them.

Black enameled Suction Chambers are in fact listed for Mark V Jaguar, in one of my several period SU Spare Parts four-page illustrated sheets, albeit without checking, cant recall exactly which Mark V model/year.
But see attached photo - albeit restored, but you have to assume probably correctly given the standard of restoration…………

Sorry to ask a question as a lurker on this esoteric list. But the photographic example of the pre-XK SU vacuum chamber provides perfect background for my question.

What is the advantage of the “neck” that support the dashpot piston? I hadn’t realized that it was so prominent in these early cars. Seems like the evolution has been to shorten it with time, until it is virtually gone on the E-type (although about 8 mm taller on the contemporaneous Mark X). I presume this was done for bonnet clearance, but is there a diminution of performance associated with shorter necks?

Robert,
I just think its an ongoing design refinement for packaging/height reduction, and not of any greater significance re performance.
The 1930s HV series basically introduced this ‘tall-neck’ arrangement, with this ‘neck’ at the top of the suction-chamber providing a guide for the piston oil reservoir, so the dashpot-piston would dampen the movement of the piston as required. In the 1930s bonnet clearance was not an issue. SU clearly played around with brass-pistons and later aluminium pistons, with and without springs, and springs of different ratings, and suction chambers with air-bleed passages or not, to achieve the desired ‘piston movement’ damping to suit individual engine application.
The earliest H series carburetters were introduced late 1930s and immediate post-war and all still had ‘tall-necks’ but from about 1952, they were redesigned to have much shorter necks given the increasing number of low-height bonnet design cars - a major issue for XK120 originality with their twin H6 carburetters. But I am sure the suction chamber/piston design change was still carefully conducted to achieve the desired piston damping. So year for year, car for car, I doubt there is any functional difference between long and short neck suction chambers, but there is most definitely a ‘packaging’ benefit with less and less bonnet clearance. Clearly same situation for E-type HD8s where bonnet clearance is minimal, but no impact on carburetter performance.

Roger

Thanks very much, Roger. It is gratifying to hear that opinion from someone with your expertise. Clearing the bonnet of my S1 XJ6 required the E-type height. I had always wondered about this evolution. Interesting about the springs and air bleed passages. Perhaps those modifications were to compensate for reduced neck height.

Pree war the dampers were brass and relied at least to an extent on weight for dampening.
Postwar for ease/cheapness of manufacture they went to aluminium dampers which because they were lighter needed springs. Or that’s the simplified reason as explained to me by the SU carb maker.
The other type of suction chamber top was the beer bottle shape which seemed to have been used on SS 100s[ and a few other sports cars but not saloons. And I’ve never heard a reason why.

I can see the sense in having black tops in a Bentley that has a black rocker cover but it just looks misplaced in our cars that sport alloy covers and manifolds.

Peter

I suppose black rocker covers too
I think that apart from looking shiny, the alloy rockers also insulated tappet noise from owner’s delicate ears.

Roger’s photo is of a Mark IV. The air intake plenum and filters and location of the fuel pump are characteristic. Unless it was one of the very first few RHD Mark Vs. The air filter was changed because of LHD.

Here is an H3 from 1938 with tall piston chamber stem. There is a gusset on the side but no hole drilled in it. Brass piston but no spring or damper. The cap says USE BICYCLE OIL and I guess you need an old British coin to remove it as our US 25 cent piece does not quite fit the slot.

Here is an H4 from 1950 with shorter piston chamber stem but no gusset nor hole drilled in it. Brass piston, no spring, damper on the cap.

Here is an H6 from 1952 with shorter piston chamber stem and side gusset, and notice there is a hole drilled through the gusset. Aluminum piston, spring and damper on hex cap.

I believe that the reason for shortening the stem was in order to drill the hole in the gusset. You can’t get a drill bit in there with the tall stem.

Here is a paragraph from “Automobile Engineer’s Reference Book” published 1956.
“On the majority of SU carburetters manufactured since 1950 an air bleed has been added to the main jet assembly to assist mixture stability under certain conditions of throttle opening.”

Very interesting! On the 1950, is that a vent hole in the cap, centre of the coin-shaped slot? Also, is the oil there for lubrication of the piston (since there is no damper)?

Regarding your theory about the neck being shortened to provide access for a drill bit on the gusset: Some of the later HD8s, IIRC, lack drillings into the gusset and instead rely on a vent hole in the cap. Could this method have been used instead of shortening the neck?

here is a 'beer bottle top" SU from an SS100

and for a rare oddity , a twin choke SU [ 2 x 1/34" from memory.]

WE all need to be a little bit careful being to simplistic and generalist with SU carburetters, especially isolated spare-parts components of unknown origin.

The ‘H’ series was a family of carburetters that came in several sizes and a myriad of different specifications, let alone evolving component design/manufacture over their late 1930s to late 1950s period of manufacture, and now also modern reproduction parts, bearing in mind they were used by many more manufacturers, other than just Jaguar .

But essentially you had a ‘H’ style main body design (in H1 to H8 size), onto which all manner of different components could be built up to suit the exact end-requirement of the various car manufacturers to suit their different models and different engines.

There were several different sizes and designs of Suction Chambers, and indeed Damper Assemblies (with their round or hexagon, plated or unplated, or vented or not caps), and indeed Suction Chambers of differing diameters, venting arrangements, mounting arrangements, damping arrangement, could be specified as being enamelled black, left as cast, or be ‘polished’. For instance, the basic H6 carburetter as fitted to XK120/140, actually had fourteen variants of the main body casting sub-assembly, of which only two of these variants (/1 and /4) were ever used for XK120/140 - not counting the additional significant age related changes made 1949 to 1957.

How SU handled this is by providing very detailed illustrated and detailed 4-page (usually) Specification sheets, where for a particular Make/Model/Engine application, each individual component in the entire (in Jaguar XK case) twin thermo-H6 set-up, including linkages was detailed, including in most cases, advise on components being black-enamelled, Walterised, plated (DN or Cad), or polished.

There are EIGHT different SU Specification folders for Mark IV Jaguar, and FIVE different for Mark V, so it takes a bit of effort to sort it all out, let alone being to emphatic about individual loose components in isolation . The Jaguar publications detail is minimal, simply at an assembly Part Number and serviceable parts level.

Unfortunately however, factory SU information on pre-war SS-Jaguar carburetters is minimal.

My ‘rents’ PIII R-R had the entire engine enameled black.

Dad hated it, and took three months to remove it all.

The hole in the slotted cap of the 1938 H3 is for lubrication of the shaft, which is not drilled for a damper. It vents the small volume above the shaft end, but does not vent the large volume above the piston, as does the angled hole in my 1952 H6.

I think a sixpence might be the factory recommended tool

Rolls Royce owners, of course, used a crown or guinea.

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Surely Rolls Royce owners have their man use a sixpence.

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I doubt a Rolls Royce owner would trust someone in service with a sixpence.
Closer to home, SS and Mk IV D lenses used a sixpence { 6d] to undo the knurled screws, as did the screw on the SS100 owls eye tai.
OMG now I’ve started a whole discussion on the correct coin in the SS100 tool kit.

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Hi Ed,

…and what is the oddity in the centre of the engine photo running on the fan belt?

Peter

I think David Bowey said it was a " space oddity"
But if anyone has any theories,I’d be pleased to hear them
The owner owned Trico , if that’s any clue.