idling speed with SU HD6 carbs

[Barry153]

Hello, I have a 1964 Mk2 with 3.8 engine. On a few occasions recently the car has refused to start although cranking over as normal. I eventually found that the piston in the front carb was sticking so decided to strip and clean both carbs.At this point I should say that I have never done this before!
following the workshop manual and U Tube I dismantled, cleaned (Very dirty) and reassembled the carbs. Used a Gunson balancer to balance the carbs and then a Colourtune to set the mixture.fitted the air cleaner (New element) and started the car.
On automatic choke it turned over at 1000 RPM, off choke it was at 500RPM, as it warmed up towards normal temperature it idled at a very satisfactory 750RPM,Wonderful! On the road it was wonderful, much livelier and pulling smoothly from 1500RPM. I was delighted.
Today I have done a run of about 50 miles and towards the end the tickover was at 1000 RPM!
It would appear that as the temperature increases so does the tickover speed.
Can anyone give me any thoughts about this?
Thak you,
Barry 153.

[piman]

Hello Barry,

I know the Mk 2 has a very large air cleaner but the mixture should really be set with the air filter on. I think you should be able to read cylinders 2 and 5 with it fitted? How many colour tunes do you have two would probably be better than one.

When the engin eis idling, it really is not up to full temperature despite what teh water temperature gauge says. Try resetting teh idle to 750 after a run and see how it woks from cold. I take it you have a manual car as 750 rpm is correct for that, an automatic should be set to 500rpm.

Alec

[Barry153]

Hello piman, thanks for your thoughts but in my long ramble I omitted an important point. in order to get the idling speed to 750 RPM I had to screw down the idle adjusting screws down to the stop so I can't reduce the idling speed with those screws.
I take the point about adjusting the mixture with the air cleaner on and I agree that cylinders 2 and 5 can probably be reached but I don't think that i could reach the mixture adjusting screws with it on.
Barry.

[piman]

Hello Barry,

if your idling screws are fully in then the throttles must be too far open at idle. The Jaguar set up has the linkage set up to be, from memory about 1/16" inch below level. If you have a manual this is illustrated, other wise it is rather difficult to explain.
If you do not have a manual, let me know and I'll dig mine out and try and post a picture on here.

Alec

[PPV]

It sounds like your throttles are open slightly letting more through at idling. Have you checked the intermediate throttle stop position is not holding them open. If you have not done this before are you sure you adjusted the slow running volume screw and the mixture adjusting screw and not the throttle stop screw instead of the former?
The diagrams in the manual on page C17 and C21 detail this but I had met someone who was adjusting the wrong screw in the past. I also trust you topped/checked the oil in the dashpots.
Regards,

[Barry153]

Thanks for replies. I have now looked at the relevant pages of the manual and i see what you both mean about the linkage set up.
i have been using the individual idling screws and mixture screws but before i started I completely backed off the intermediate throttle stop thinking that the idle speed should be set on the carbs!
i think I might be able to have a go at setting it up now.just two further questions, would you suggest that I turn the idling screws back two and a half turns from the hard down position that they are at the moment to restart the setting up process? secondly, how will I know when the carbs are fully closed?
As I said, I am doing this for the first time!
I am very grateful for your help.
Regards,
Barry.

[ChrisDixon]

I've had a similar issue on and off. I've put it down to the engine conducting heat to the linkages and causing some expansion, so ended up arriving at a compromise. But I think you need to sort the base settings, as others have said it sounds like the throttles are not closed at idle. There should be no gap, as the idle air is provided by bypass air passages which the idle screws act on. The rest of the linkage should therefore be adjusted to allow the throttles to be fully closed at idle and just fully open with the pedal to the floor.

If you have not fully rebuilt the carbs, and only cleaned them and fitted new gaskets etc, you may find that air is getting in past worn throttle spindles or discs, which can affect the idling.

It is also worth checking the ignition timing, and points if you have them, as this will also affect carb behavior. As a general rule carbs should be the last thing adjusted.

[PPV]

Hi Barry, you should slacken one clamp bolt on the coupling between the carbs before adjusting then rotate the throttle spindles so that the throttle butterflies are closed (might need to adjust the throttle stop screw to give a bit of clearance as per the manual). Then retighten the clamp bolt.
I peresonally do screw in the slow running volume screws until they are down and seated. Do not screw too hard as you can damage the seat/screw. Then unscrew each one 2and a half turns.
I also ensure before starting that thetop of the jets are flush with the jet bridge in each carb. This can be seen best by removing the dashpots (3 screws and take care when lifting to keep the piston and spring inside the dashpot if you are not familiar with the arrangement). When level then screw in the mixture screw 3 and a half turns and you should see the jet go down about 2 or 3mm. When putting dashpot back on the the piston has a slot in it which means it can only go in one way and keep the dashpots in the same orientation.
The carbs are then ready to be tuned once the engine is run and up to temperature.
It is easier then it sound once you have done it.
Best of luck,

[piman]

Hello Barry,

did you remove the throttle discs when you overhauled the carburettors? Refitting them entails assembling, taking care that they are in the right way as the edges are sloped to fit the throttle body. Before tightening the screws you need to press the disc such that they fit the throttle body with the minimum of an air gap, the smaller the better. The holes are oversize to allow for this, unless this is done carefully the throttles will not shut properly unless you are extremely lucky. From the S.U. manual the setting for a HD carburettor is the slow running valve 3.5 turns up from fully screwed in. The mixture adjustment is 2.5 turns down from the jet top being flush with the bridge. You really should not have to deviate much from these settings, if you do something else is not as it should be.
The throttle return springs should keep the throttles closed and it should be possible to stall the engine by screwing the slow running screws fully down.

Alec

[Barry153]

Thanks PPV and piman for your suggestions. I have now had chance to try again but i am afraid I have not been successful!
I adjusted the single throttle screw to what I thought was the 1/16th between the centres of the fulcrums,turned the slow running screws right down to their stops and then back by 2/1/2 turns and then tightened them equally to reduce the idling speed to 750 RPM. guess what? the screws reached their stops and the RPM was only down to 1000RPM!
I have decided that I need a carb expert and it is booked in shortly.
Thank you once more for your help.
Regards,
Barry.

[piman]

Hello Barry,

are you sure you are trying to adjust the right screw? The idle speed is an air bleed screw in the body of the carburettor, it does not act on the throttle shafts. It is low down and in front of the part where the throttle spindles enter the carburettor, on the right hand side of the body looking from the inlet end. On the opposite side is the mixture adjusting screw in the little lever that ccomes from the base of the carburettor?

Hd carburettors are different to most other S.U. variants, in mixture and idle adjustment.

Another thought, are the pistons free to close of properly, i.e. if you lift the dashpot with a finger in the inlet, do they both fall back down steadily and come to a stop with a slight 'click'?

Alec

[PPV]

Hi, I see you are not that far from where I live. If you don't mind me asking where are you intending to take the car to? I could pop over and take a look for you but am busy until about the end of the month if you can't get it sorted. I am not promising anything but I am familiar with the set up etc.
Regards,

[Barry153]

Hello Piman,
The situation is, I have booked the car in to TR Bitz at Warrington on November11th. I have dealt with this company for some 20 years as I bought my TR5 from them at that time. I know the TR5 does not have carburettors but all the earlier TR's do and John Worsley also repairs E Types,Ferraris etc.
I do indeed live Near Nantwich at Faddiley and appreciate your generous offer to have a look at it Are you sure you don't mind? It appears to be over and above the call of duty!
Regards,
Barry.

[Barry153]

Sorry PPV, I accidentally addressed my previous reply to piman and not to PPV. Must be old age!
To piman I would confirm that I am sure that I am adusting the correct screws as described in the workshop manual and after I had cleaned the pistons they dropped quickly with a clunk. Thanks very much for your remarks
Regards,
Barry.

[PPV]

Hi Barry, if you are travelling to Warrington there is also the Jag specialist, R G Bate Engineering in Birkenhead Tel No 0151 653 6765. I have never used him but there are good reports in Jag circles and he used to write in the magazine on tech items. If TR Bitz can't fix let me know but not sure if I could better than them as they are good.