I balanced the 3 SUs, running nicely, but couldn't get idle speed below 1200rpm (screws were well away from the arms).
I needed the transmission serviced so resorted to a local guru for carbies too - because its a shoehorned 4.2, the linkage isnt normal.
Apparently, my the linkage was fouling on the manifold so the butterflies werent closing - what an idiot, I should have been able to work that out!!!!
For those in Adelaide South Australia, Geoff Mockford is kinda a Jag guru.
I will pick up the car tomorrow and take the second credit card that has the higher limit......

 

Yes ~ because SU's idle with butterflies closed & on their own idle bypass circuit set by screw. They are still idling on the main jet but the butterfly bypass is the idle setting ~ NOT butterflies partially open like conventional carbs. (A). Obviously idle mixture must be set correctly. (B).

 

Ah - so mine being open would have wreaked havoc!!

 

AFAIK, only one of the idle screws on a multi-SU carb setup is actually setting the idle at any given time. although it's true that each carb has its own idle screw, once the carbs are linked together by the common throttle shaft only one of the screws (the one that is actually touching its boss) is actually keeping the butterflies from closing completely. the others COULD do it but in practice are doing nothing but adding moral support, so to speak, to the one that is.

 

Not my understanding but you could well be right. Screw A bypasses the butterflies. The butterflies are closed. Screw A controls an air bypass channel on HD carbs. I'm more than willing to learn.

 

roger that.

i was recently watching a video by an american SU carb rebuilder of some renown, Joe Curto. in this video he was rebuilding SU carbs and most of the video was centered on installing new bushings, butterflies, springs, and levers on the the throttle shafts of the carbs. i was in the process of buying new Burlen SUs for my MGTD and wanted to know a bit more about them. very informative.

i'll have to take a look at the SU's in my MK2 once i get i back and see what's what. SU made a LOT of carbs and while i'm fairly certain they are all similar, i'm certain they are NOT identical. TBC

 

Joe Curto is a legend on SU carbs but this does not cover what I'm talking about. Until I restored my Jag I was an absolute novice on Constant Depression carburetors. I was a Side Draught Weber man from my 7 Alfas that I had owned in my youth pre fuel injection.
This has been my bible on SU carbs. I might have misconstrued what it says on HD carbs. I have always thought screw "A" to be a tune-able air bypass to the butterfly.

 

it may well be, i'm not confident enough on what i do and don't know about HD carbs.

i've read that of all the various types of SUs only the HD has an air bypass valve and screw but on use with multi-carb setups the screws should be turned completely in and left that way, instead the two throttle shaft screws should control the idle via the butterflies. of course, this is internet advice so i won't feel confident of this until i get my car back and can see for myself.

as soon as i get my car back, i'll be digging around in there soon enough, i expect.

 

I don't very often get to tell you things Glyn , but that's what screw "A" is for.
Down Draft, Up Draft, Side Draft, Depression carburetors, they are all venturi's and all work on the same principal, the SU's and others like Stromberg's simply have one jet where the flow of fuel is controlled by the air pressure drop in the venturi.
The drop in air pressure raises the piston the more you open the butterfly valve (step on the gas).
The flow of fuel is controlled by the taper of the needle in the main jet, and with out the needle, there would be no metering of the fuel flowing into a given engine.

The flow of air (atmosphere) into the engine is controlled by how far the butterfly valve is opened, or in the case of screw "A", the butterfly valve(s) are now closed and the flow of atmosphere (and fuel) is solely controlled by how far screw "A" is opened or closed.
This makes it much easier to set the idle and balance when more then one carb is added.
Technically though, the carbs are only critically balanced from the use of screw "A" at idle; when the butterfly valves are opened to drive the car away, the carbs will only remain balanced as well as how the butterfly valves fit in the throat of the carbs. _ there will always be air leakage on the best seated valve in a carburetor, but it's impossible to notice if they are out of balance because the engine is running fast enough where one doesn't notice.
Setting all the butterfly valves right shut is good enough to have a balanced multi carb set-up when driving down road once the valves are opened by your foot.

What I'm about to say Glyn you probably know, but for the sake of others.
Carbs that are not like SU's, are called "fixed jet", they have different sized jets in the throat of the venturi, and as the butterfly valve is opened, the flow of air draws fuel into the venturi where it's mixed with atmosphere and burned in the engine.
The further the butterfly valve is opened, the more jets come into play.
If you ever looked inside the float bowl of a fixed jet carb, and you see all these removable brass screws in there with tiny holes in them _ those are all the fixed jets in that type of carb.
If one wants a little more fuel to flow into the venturi, one would choose a different jet with a larger hole. OR if one wants a bit better fuel economy, one would choose a smaller jet.
My Grand Dad had a 56 Monarch and to get better fuel economy he placed a piece of fine brass wire in the jet that was used to cruise down the highway, this blocked the flow of fuel just enough to get better economy, but not so much to notice the lack of fuel.

 

Not on the HD type of carb. The slow running screw is a bypass port that lets air go around the throttle plate, so each carb needs to the have the airflow balanced to be the same as the others. The throttles also need to be synchronized, so each opens at the same time and all the cylinders can draw the same amount of mixture.

The HIF type for example, does not have the bypass port, so the idle screw does open the throttle plate. If the throttle linkages are synchronized then the screw on one carb should open the others an equal amount. In practice it doesn't quite work that way due to the freeplay in the linkages, so the individual idle screws do need to be adjusted to get the same amount of air going through each carb.

I am currently working on a 1972 V12 E Type that has had the HIF carb conversion, and getting the linkages all synchronized and each of the 4 carbs drawing the same amount of air is a major pain.

 

Thank you JB. I've gone back and read it again and that is still my understanding.

Thanks for the input Jeff. Most of which I am well aware of.

 

Not all HD carbs had the by-pass valve screw, my 51 Bentley does not, balancing and idle was achieved by solely adjusting the butterfly valves.
It had this odd linkage between the two carbs where this was done, and it was a tricky thing to carry out.

Just to clarify, screw "A" is not just an air valve, fuel is also flowing through there, it's the only place it can as the butterfly valve(s) are shut, or so they should be.
Fuel and air is diverted through a small hole just after the main jet in the throat of the carb and just before the closed butterfly valve.
If you remove one of the SU's and look at the end that's bolted to the intake manifold, you will see another hole where the fuel air mixture flows around the closed butterfly valve.
What can go wrong with the screw "A" system is that the O-ring that seals the screw fails and you get an air (vacuum) leak, this makes it impossible to tune the car at idle because of unwanted air getting into the engine.
Another problem is if you get Godzilla mechanic in there who over tightens the screw on its seat and destroys the taper on screws "A" _ this also makes it impossible to tune.

About what you've read with the screw turned in all the way, this makes no sense to me at all, it's defeating the purpose of ease of tuning at idle.
I can't imagine trying to balance 3 SU carbs solely using just the butterfly valves, but I have had no experience with triple SU's, so maybe that's how it's done, but I just can't see that.
There would have to be quite a complex linkage to achieve "balance" on 3 carbs.

There is an exception to this when it comes to a Silver Shadow Rolls Royce, screw "A" is turned out 3 turns on both carbs and then the remaining of the balancing procedure is done with the butterfly valves.
I have not looked for an explanation for this, but I suspect it due to the size of the engine VS air flow.
The smallest engine in a Shadow is 6.2 liters and later a 6.75 litre, that's a lot of air pressure (atmosphere if you like) flowing into that engine, I think the engine would be starved for fuel and air just using screw "A", and/or if one could get enough air flow through screw "A", the pull of the engines displacement on the butterfly valves would draw them open _ it would take a very strong return spring to prevent this.
So screw "A" is opened by 3 turns.
The Shadow's (without checking) use HD8's, but there's still only two of them, so lots of air flow just through 2 carbs.

With Jaguars triple carb set up, I can't see where screw "A" would not get the job done just on its own.

 

It's not bad on 6 cylinder E Types. There is a single shaft on the intake manifold connected to the throttle pedal, and then linkages to the throttle (butterfly) shaft on each carb. Simply loosen each linkage at the carb and make sure the butterfly is closed. There is a return spring on the other end of the shaft on each carb, so the return is fairly positive. Tighten the linkage at each carb, and the butterflies are set.

Then open the idle screws 2 turns from the stop ( or whatever the manual says), start the engine and adjust each screw until the airflow is equal in each carb and idle speed at at the target value. It's actually fairly easy to do.

 

as i posted in #8 "...and don't know about HD carbs." i was speaking in that post WRT what i assume are the standard SU carbs and setup on a MK2. i'll be researching this and when i get my car back i'll have more confidence that my responses contain a modicum of accuracy.

 

That is simple and it confirms that all 3 throttle valves are closed and screw "A" is used for idle and balance.

 

On the Jaguar version of the HD8 it is. Interestingly the version of the HD8 used on Aston Martin DB5's does not have the screw A, that hole is not drilled in the casting and the idle speed is adjusted with the butterfly screws.

 

 

These days it's much easier and accurate to use a wideband AFR gauge in the tailpipe. I've been doing a lot of tuning on the E Type I mentioned earlier, and it's so much easier to see what is going on with one of these gauges - especially when trying to get the two banks running the same. It gives a lot of information on a short roadtest for things like acceleration enrichment that is very difficult to determine otherwise.

https://www.aemelectronics.com/produ.../parts/30-4110

 

The crude old ways we used to do things JB. In the early days of detergent fuels we had the leading product & still do to this day & I used to carry an AFR tester, exhaust gas analyser around in the boot/trunk of my car. A little larger than yours and it took ages to calibrate before use.

What I'm pleased about is that my original answer to the OP in post No 2 was correct for HD carbs. I would hate to mislead people here. I have not which pleases me as I'm my own worst critic. Why you see so many "refresh page for edit" from me. I like to be precise in what I say.

 

I think the idea is to ensure each carb is separated, adjust each one's idle, link back together and set overall idle speed from the main linkage. On mine its easier said than done !!!