NWG:
I never made the connection of over-tightening of the serrated washer to the point where it would get crushed _ and the solution was to add another washer. (I read your post several times)
If I caught my mechanic over-tightening the float chamber nut to the point where the fibre washer was being crushed, I'd smack him in the head !
From what you describe is the reason why I don't let most others touch any of my vehicles, even my daily driver.

And yes, my Bentley I believe has two per carburetor because of the added breather device that is connected directly to the main jet _ a rather strange arrangement to look after a flat spot in the power curve when RR was doing trials on the track _ or so I was told.
I've disconnected the devise as in very hot weather, the car would idle poorly when the device was active.
Since then I have replaced the carbs with some newer SU's that do not have that feature.
Rolls Royce loved to make things needlessly complicated.

Here's more on that subject, it mentions that no other manufacturer uses the device, but I think Rover did at one point.

Australian RR Forums: What Is The Point of The Weakener ?

 

JeffR1
Just interested that you read the Australian forum on the subject................I know about it as I wrote the answers!!

 

Thanks Norman, I would have not made the connection.
And that's me over there as well Jeff Martin, I started the thread.

 

Norman your post and your response made a little research necessary. The following is Jaguar only as we have no real interest in RR and Bentley. We believe perhaps your confusion comes from your experience with RR and your miss-reading of the Spare parts Catalog(ue) or someone else miss-reading and relating that to you. The WSM (work shop manual E/133/3 of 1972 has the same parts illustrated as the SPC (Spare Parts Catalog (ue) J35 of Aug 64 Both for the 'S' type.. There are no bulletins on the subject except the lid change and 2 needle and seat changes. When you study the SPC the front and rear carb parts list call for 2 AUC 1928s and 2 AUC1557s per carb. In fact what has happened is all small parts listed are called out in car qty not unit qty. This is an obvious mistake and the only parts that have an accurate qty is the larger main handed parts. This is perhaps not convincing enough so we opened an original SU (not Burlen) repair kit for an HD6. One each AUC 1928 and one AUC1557 in the kit. The kit was from circa 1969 as it still has a Royston Label (dated) on it.

So in this case the Jaguar "specialist" are doing it right. Tomorrow we will check the SPC for triple SU cars and bet we find for the small parts it calls for 3 of each for the float chamber. We considered Jeff's observation re stacking and staggering issues and can't help but believe that if this was an issue SU would have simply stamped a thicker washer.

You do bring up an interesting point re: overtightening. Other than having a poor gasket under the float lid why would that happen. If all is proper a simple snug works as there should be no fuel in that area (liquid) unless under VERY spirited driving. Two washers would rotate as anyone that has tried to keep the vents from moving will attest.

It is important when making a claim of technical change that it be supported with documents or such. Although Jaguar were/is a small company they did do a very good job with documentation. In this case the SPC is at error but is more of an SU mistake at first and a Jaguar secondary issue.

 

NWG seems to be all over the internet. Here from Jag Lovers.

"I can offer a particular scenario that may be at the root of your problem, it has certainly been the case in the past. I think you will also find it is not in any book, or this forum.I believe your overrun enrichment has been caused by your attempt to tune the carburetters for acceleration, while the float chambers are under depression.

This can be because the overflow pipes are blocked or undersize. However it is usually because the internal serrated fibre washer S,U Part AUC1928 (Jaguar Part 1298) positioned between the float chamber lid and overflow pipe has been squashed, not fitted or there is only one washer on each carburetter.

That is to say that there should be TWO AUC 1928 stacked together on each carburetter. This arrangement was always stressed at the S.U service school. Unfortunately, as far as I know the Jaguar parts manuals were never modified, other operators like R-R, Bentley and Aston made sure that detail was contained in their parts manuals.

Those internal serrations on the AUC1928 are the only path for air to vent the float chambers, if they are crushed by overtightening that air path is further reduced resulting in fuel starvation. In some instances, particularly in North America dollar area, the special serrated washers have been replaced with normal fibre washers, blocking off the air supply. Remember, any owner, or mechanic will receive rebuilt carburetters in part disassembled form and that owner or mechanic will be the one to make the usual fatal error when they attach the overflow pipes. Unfortunately, they remain under the impression that their carburetters are beyond fault……after all they have had specialist attention.

On larger engines like the Jaguar XK the air restriction demands an extra serrated washer is fitted, or even a third washer. The advice at S.U, in the days when technicians knew how these carburetters worked, was to file a “flat” from the underside of the hexagon head on the float chamber cap nut, to form a “D” section. The “flat” section extending down the S.U Part AUC1867 (Jaguar Part 5832) cap nut to the inside of the float chamber lid. This provides a permanent air vent and I believe was a standard feature of Rally cars.

You are frankly wasting your time trying to tune any S.U carburetter without two of these washers on each carburetter and most certainly trying to obtain 14.7 ratios with any carburetter. Those units were never specified using that ratio when new. As someone previously stated they have to run rich on acceleration. Obtaining needle sizes and power output was not done by trying to adhere to 14.7 ratios but by forming power loops to obtain the best MEP and WMMP performance.

I hope that my explanation of a particular scenario may prove helpful, if not, it may explain to some other owner why their car is down on power."



As a petroleum man
For gasoline as a fuel, the stoichiometric air-fuel ratio should be about 14.7:1 ~ best for emissions not power! Not where one typically runs.

For maximum power in modern vehicles your target air-fuel ratio should be approx 13 to 1
For older less efficient vehicles an ideal AF ratio for power would be about 12.5 to 1

As you come off the gas the A-F ratio will lean to the other side of stoichiometric. Some Injected vehicles shut off fuel completely on coast.

With XK engines I find maximum power is achieved running just marginally rich. A little brown wisp out of the exhausts on acceleration that quickly clears. Probably the fuel pump effect over the jet as the damper lags in it's rise. This is running standard TL needles. (S Type)

 

I found it quite interesting watching how much the A-F ratio can change while driving, by hooking up a wideband gauge. I usually tuned my carbs for about 14:1 for steady state cruise, which translated to 12.5 for heavy throttle power. Idle often ended up being around 13.5. Rather than going lean, my gauge often showed rich under closed throttle coast. Could be the gauge misreading lean for rich, or lean misifre too.

 

Hhmm, my question has spiralled into this :-)
I put one gasket under, and now that it's the right one, y problem is solved. If anybody suggest to put 2 instead of 1 to compensate for people overtightening, it would be easier to not overtighten...

Thanks for input from most, did solve my issue

 

Yes ~ AF Ratio (consumption) moves all over the place dependent on throttle position & terrain. I enjoy watching it real time on my Merc but it's like all things Merc ~ it's heavily damped unfortunately/fortunately. My Holden Berlina company car in Aus was the reverse (underdamped) & howled up & down with ratio changing so fast you could not read it. So that was generally useless under most conditions. All I knew is that I averaged 19.1 litres/100Km over 23,000 Km that I ran on the car from new. Horrendous! Too much crawling in traffic & the odd WOT in the country. That Harbour Bridge every morning. By the time I left the office in the evening the traffic was gone. (3.8 litre V6 Ecotec.)

Your numbers look pretty much right although I would expect more in the 13.5 range at medium throttle. (But we might actually be talking the same thing ~ steady throttle)

Can't explain your off throttle/coast readings but I have not seen your exact set up or SU's idle circuit effect on it with butterflies shut but revs still on the engine. What are we really reading?

 

We checked the MK10 and the E type SPC and it appears that the project managers at SU and Jaguar got these correct--one per carb. That said we left Jaguar and went directly to the 'limited" SU sources we hold. We have (we think) each version of the SU factory workshop manuals up to vol. #15 (by Burlen). There is no ref. for stacked washers to improve power--rather they take the normal approach. Note the kit qty is called out.

 

Classic engines were usually tuned to run a little on the rich side at idle with an ignition timing a little retarded from optimum. That's why old engines on carbs often idle much more smoothly than more modern ones with EFI.

The Series 3 XJ computer responds quickly, but not so fast as to become meaningless. Although it's based a calculation of fuel used, it truns out to be surprisingly accurate for average fuel consumption.

As for the SU washers, it's a classic combination of British design, made on the assumption of that all engineering fitters are R-R standard, meeting the man with the giant spanner, who over tightens anything he touches. Although it's not purely British. the gasket on the oil sump of the GM 400 gearbox is always over compressed by anyone without a manual and a torque wrench.

 

Now that I clearly have found some SU experts in here, I'll take the opportunity to ask this: How do I recognize which SU I'm dealing with? I work on Mk2s, XJ6s and E-types S1. And sometimes , like on my current XJ6, parts including carbs have been replaced. So I need to be able to recognize what I'm dealing with.

Am I correct in the following:
HD: Side float chamber, one-nut banjo on the top
HS: Side float chamber, two-stud, three screw on the top
HIF: Bottom float, and none of the normal adjustments possible, emissions/US carb

If I'm right, then which ones used the Auxilary Enrichment Device (using a wire to a sensor), and which ones used the Automatic Enrichment Device (using hot air from exhaust)? And what's an ASC, Auxilary Starting Carburetor (mentioned in my Jag book, but not in my SU book)?

Thanks

 

I think I can answer this part, I believe the ASC (Auxiliary Starting Carburetor) and the Auxiliary Enrichment Device (using a wire to a sensor) are the same thing.

 

HD: D stands for diaphragm. This has a flexible diaphragm around the main jet, with fingers from the choke mechanism that push the diaphragm ( and thus the jet) downward to enrich for starting. Metering needle is rigidly fixed to the piston. Used on Series 1 E Types and Mark 2/S Type, except for the earlier 2.4 Mark 2 that used a Solex.

HS: Has an external flexible tube to supply fuel to the jet, so has a distinctive appearance. Fuel supply to the HD carb's jet is internal in contrast. The metering needle is spring loaded to bias it to one side of the jet on some models. Also has a float chamber to the side, like the HD. I'm not aware of Jaguar using the HS series.

HIF: Stands for Horizontal Integrated Float. There is no separate float chamber, the float is in the bottom of the carb, and the fuel supply to the jet is internal. Metering needle is always spring biased. For starting enrichment, there is a separate rotary valve to admit extra fuel, the main jet is not lowered like in the HD and HS series. All the usual adjustments for air/fuel mixture and idle speed are possible. I wouldn't necessarily call it an "emission" carb, but it is more refined than the earlier carbs, and in my experience does an excellent job of maintaining the A/F ratio. If I was not interested in originality, I'd use HIF's in a carb application. In Jaguar application for the US market on E Types (from 1968) and XJ6's they used Strombergs in place of SU's for emission reasons.

 

 

 

I guess I should clarify that when I talked about the jet being lowered, I am referring to manual choke applications. The Mark 2/ S Type have an Auxiliary Starting Carb in between the main carbs, and it is controlled by the thermal switch in the intake manifold and a solenoid to turn the carb on and off. It provides extra fuel for starting and shuts off at a predetermined temperature, usually 35C.

Later cars, like Series II XJ6 had an Auxiliary Enrichment Device, which performed a similar function to the earlier starting carb.

 

Allow me to pose another SU question about an issue that popped up today. My XJ6 S2 has had an engine transplant, still 4.2 with twin SUs, but they are HIFs and I'm told US-spec, because some adjustment screws are sealed. They run fine around town. Had a hard time starting it when warm, switched AED with the one from the old engine, adjusted it, so now it works fine, starts easily. So took a long trip today to test, including freeway driving. And doing that at 60-80 mph for a while, it started to stumble/pop, clearly missing fuel to support it. Both tanks have fuel, switching tanks doesn't change anything. Also I do believe it gets enough fuel to the carbs, this stalling/popping with accelerator 2/3 down happens regardless of whether I've just driven at high speed for a while or slow speed. So I don't think it's float chambers that run low on fuel.

So what can it be? This SU carb is a single step carb, right? If it wasn't I would think step 2 didn't work. Could it just be dirt in jets or something?
Any ideas before I take the two SUs apart the next days?
Thanks

 

Sounds like fuel starvation and it's going lean. I'd start with a fuel filter change, it could be blocked. Also check that the fuel pump is providing enough fuel. I'm not sure what the rate specification is supposed to be. No dents/kinks/ other restrictions in the fuel line?

The "Sealed" carbs simply have little metal caps pressed into the bores where the adjusting screws are. Pry them out, and the screws are exposed and you can adjust as normal.

 

As I wrote, I don't think it's up stream fuel starvation as both tanks/pumps do the same, and it doesn't matter if I went fast or slow just before. It has to be SU internal I'd say. But what?

 

Don't be so quick to write off fuel supply. The tanks are interlinked & just a minor drop of fuel pressure at any speed/load can cause loud popping, misfiring, backfiring, stumbling etc. Remember these fuel systems run in the 1.5 to about 3 psi range.

This air leak on the left tank caused mayhem on either tank or pump where there appeared to be good pressure up front. Had us foxed. Seemed to be plenty of fuel at the fuel filter & carbs. But the engine would pop, backfire & stumble when loaded in numerous conditions.

Carefully check your fuel supply from rear to filter even if all seems fine up front with plenty of fuel & a full glass bowl (well as full as they get)

This was still semi pressed on the metal pipe & no smell of fuel.