HD8 Carburettor - choke adjustment

ChrisC · Post by **ChrisC** » Sun Jun 17, 2012 5:05 pm

Folks,

My car is running a little rich and i think it is because i have messed up the choke settings so I want to check my understanding with you learned gentlemen.

I can see that the choke cable pulls on the mixture control rods which then pull on the jet lever of each carb until it touches the fast idle lever and continued opening of the choke then opens the throttle slightly.

My problem is that on each carburettor the amount the jet lever has to move until it touches the fast idle lever varies from one carb to another so basically one carb opens the throttle before the others... The various levers are set in position on their spindles by being drilled and pegged so there is no adjusting that.

So......

Option 1 - do I adjust the mixture control rods so that they connect to the jet levers when they are totally at REST and live with the fact that 1 of them operates the fast idle lever before the others or

Option 2 - do I adjust the rods so that they partly operate the jet levers of the other 2 carbs so that continued choke opening then moves ALL the fast idle levers at the same time?

Gut feel says option 1 is correct and there is nothing i can do about 1 carb increasing the throttle before the others...

Advice please? I can't find this any literature.

Cheers

christopher storey · Post by **christopher storey** » Sun Jun 17, 2012 6:32 pm

Chris : if your setup is operating the jet levers before it opens the throttles then there is something very wrong with the adjustment , and I would guess that it is because the throttle opening bits are not properly screwed together. There is an S shaped lever which is fastened ( with a spring underneath it ) to the top of brass rod which has the shoe and cam at its bottom end, operated by the fixed linkage . If it is not properly seated on the brass rod you may get the effect you described. ( Ditto if the brass rod is partially seized within the body of the carb, which is not uncommon) . The other problem may be that the brass rod is slightly askew at the bottom end, so that it is not engaging the cam properly . If all is properly set up, the sequence of movements should be that the first part of the choke cable movement operates only the brass throttle fast idle rod , further movement then opening the rich mixture lever so as to lower the jet beyond its normal position

ChrisC · Post by **ChrisC** » Sun Jun 17, 2012 8:04 pm

Ah my fault I used the wrong terms I should have got the manual out sorry.

So....
Using figure2 on page C4 of the 3.8 workshop manual.

It says in the third paragraph:

The second stage brings the jet lever (A) back further opening the throttle further and bringing the lever (F) into contact with the mixture screw arm (G)....

On my car I have 1 carb where (A) moves F onto G sooner than the other 2 carbs... And this is without the carbs being connected to each other it is simply how the lever (A) is fixed on that carb. So do I make all the carbs move G at the same time ? Which means the jet lever (A) is partially operated on the other 2 carbs all the time

I am not sure what a partial movement of the jet lever (A) results in - it may be nothing or it may result in my not being able to tune the carbs properly.

Post by **Heuer** » Mon Jun 18, 2012 7:51 am

Christ

Have a look in the Knowledge Base - there are some very good SU guides there. I particularly like the period SU Tuning publication.

ChrisC · Post by **ChrisC** » Mon Jun 18, 2012 8:45 am

Thanks Dave but sadly for the life of me I can't see where any of them mention how all 3 chokes should operate together and whether all 3 jets must stay at rest or whether all 3 need to move the mixture arm at the same time or not.

Im beginning to think the answer to the question is not widely known and everyone leaves them in their home position. Which is where I think I will revert to...

christopher storey · Post by **christopher storey** » Mon Jun 18, 2012 11:38 am

Chris : you can take it that whatever operation is contemplated, all 3 carbs should work in unison. The Jaguar set up is not like eg certain Austin/Austin Healey 6 cylinder setups where one carb was set to open its throttle fractionally before the other . What I suggest you do is take all 3 brass rods out ( not difficult - just undo the screw attaching them to the throttle s shaped levers - don't lose the spring) and check they are all the same length and staright and move freely in the bore of the carb body - worth also checking that the roller in the shoe revolves - then reassemble . Make sure that as setup, none of the s shaped levers is opening a throttle . Set the jets with the adjuster screws to about 65 thou down from the bridge as a starting point , and then check that all 3 throttles, and then all 3 jets , move together when the choke apparatus is operated

mystery type · Post by **mystery type** » Mon Jun 18, 2012 6:35 pm

Christ Have a look in the Knowledge Base

A bit harsh, he was only asking.

christopher storey · Post by **christopher storey** » Mon Jun 18, 2012 6:49 pm

I think David promoted him inadvertently , although with the recent weather walking on water would be a useful skill

ChrisC · Post by **ChrisC** » Tue Jun 19, 2012 6:32 am

Lol.... i didn't notice the promotion or sense any level of harshness... :)

I think i have known you bunch of rabble for long enough now not to take any offence.

Besides the old SU book is quite interesting although it didn't have what i was looking for.

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HD8 Carburettor - choke adjustment

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