Need Assist S1 3.8 Choke Adjustment Procedure and Carburetor Tuning Sequence

Hello all, first post but 1 year reader. Having trouble locating these answers for my father-in-laws’ numbers matching '61. I am trying ever so hard to get it running for him, but cannot find answers to these questions:

  1. With new rebuilt carbs, is Choke setup first or is tuning the carbs first?

  2. For the choke adjustment/setup, what is the correct procedure?


There is MUCH in the Archives: this may help you with your question.

Tuning triple SUs.

Before you start you need to have the ignition and timing correct.

• Ideally, use new plugs and points.
• Plugs gapped to 25 thou.
• Points gapped to 15 thou.
• Ensure rotor button, cap and leads are serviceable.
• Timing set to 9 degrees BTDC at idle. Set vernier adjustment to the middle of its range to allow for later fine tuning.
• Ensure both the centrifugal and vacuum advance work.

In my view the only accurate way to set SU mixtures is by measurement of exhaust CO%. I use a Gunson G4125 analyser that costs around $200 from EBay.

You will also need to use a vernier caliper to measure jet heights.

Remove the air filter plenum.

Undo all the throttle links and ensure all 3 butterflies are fully closed.

Ensure that your throttle linkages are adjusted so that

• they will give full throttle
• do not catch on anything else
• return correctly to have all 3 butterflies completely closed
• there is the same amount of free play in all 3 actuator arms, so that all 3 butterflies are synchronised

Check that all 3 needles are correctly centred in the jets by lifting the piston with the damper removed. It should drop freely and stop with a clunk at the bottom. If it doesn’t you need to re-centre the jets. The procedure is described in the manual.

Remove the bells and pistons. Do not mix and match. Check that all 3 piston springs are the same rate.

Check that the needles are set to the correct height in the pistons. There is a groove cut in the base of the needle. Use a 2 thou feeler gauge in this groove to set all 3 needles the same.

Pour out any oil in the pistons.

Replace the bells springs and pistons. Oil and replace the dampers.

Set the fuel bowl float level to 7/16".

Bring the engine up to operating temperature.

Reset all carbs to the initial spec in the manual.

• Slow running screws. Close fully then open 2 full turns.
• Set the mixture screw by adjusting the jet on all 3 carbs so that it sits 60 thou below the bridge height. This is more accurate than using a certain number of turns as per the manual.
• Adjust the fast idle (choke) screws using a feeler gauge so that they clear their stop by 2 thou. Ensure the choke lever is fully off before you do this.

Restart the engine. Adjust the idle speed down to around 700 rpm. Check that all 3 carbs are synchronised using a flowmeter.

Using the CO meter and the mixture screws on the carbs, adjust first the left (front 3 cylinders), then the right (rear 3 cylinders) exhausts so that you have around 5% CO. You need to consider that adjusting the centre carb affects BOTH pipes and adjust accordingly. Once you have the right CO reading readjust the idle as above. This requires a little mental gymnastics at first but you quickly get the hang of it.

Then remove the bells and measure all 3 jet heights. Theoretically you should have all 3 at the same height, although this often doesn’t happen.

What I do then is reset all 3 at the average of the 3 heights, then readjust mixtures for CO as above, readjust idle using flowmeter until I have jet heights approximately equal, idle flow equal and idle speed at 500 to 800 rpm depending on the type of engine. This can take several run throughs. If you get confused and end up way off then just reset everything to the starting spec and repeat.


Choke and mixture adjustments do interact to a small extent. You can set the jets down rather low (three turns or so from flush with the bridge), to get the engine started, then warm up the engine, and adjust mixture. Just make certain the choke linkage is NOT contacting the enrichment arms while adjusting mixture. Once mixture is set, adjust the choke linkage so ALL THREE contact the enrichment arms at EXACTLY the same time. Once that is done, adjust the choke cable clamp so the choke arm JUST make contact with the enrichment arms when the choke lever is raised to the mid-point of its travel. Below that position, ONLY the fast idle cam should be active.

When adjusted properly, the engine should start from dead cold in any temperature with no more than 2-3 seconds of cranking, and should idle smoothly at no more than 1000-1200 RPM. When starting, turn on ignition, wait for pump to stop ticking, raise choke lever as needed (full up only for the coldest days), then crank. As soon as engine fires, lower the choke lever to get lowest stable idle, which should never be over 1200 RPM.

I hope it goes without saying, it is also absolutely critical for the throttles arms to be PERFECTLY synchronized, the return springs to all be working properly, and the fast idle cams to be adjusted so there is a small clearance when the choke is fully off, and matched airflows through all three carbs when the fast idle cams are active.


I replaced the linkage cotter pins(3) with small “hairpin” style clips as dealing with the cotter pins can be a true test of your patience


Thank you all for the help, I am waiting on the service manual to arrive and will coordinate your directions with the book. I am fairly certain I can get it done with your advice. I have spent countless hours reading and searching on the forums. Your help is exactly what I remember seeing and couldn’t find it again. Thank you so much!!!

Sorry to resurrect an older thread, but on this point, my car has been very hard to start, though runs reasonably well once it does start.
I’m suspecting that my choke is not properly moving the jets for enrichment, and wondered if anyone has a spec of how much they should lower when choke is full-on so I have something to check against? There didn’t seem to be anything in the manual about adjusting the choke other than the fast-idle.

Interesting question so I decided to take look. As far as I can see, without taking the dashpots off, if the lever on the dash moves through it’s full range and the cable is tightly secured both ends, the mixture lever only moves up by about 5-6mm, so the jet will drop by a similar amount.

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I have tried and tried to get full travel out of the choke lever and not succeded with the cable attached to the carb levers…Only get about 1/2 travel…but this works fine so I left it alone…Ive seen few cars where the lever goes all the way to the top position…

When properly adjusted, the enrichment will not start until the lever is half-way up. Before that, there should be ONLY increased idle speed, which should amount to ~1200 RPM (on a warm engine) at the point where the jet levers BEGIN to move.

It is ABSOLUTELY CRITICAL that ALL THREE (or two for S-Z carbs) enrichment levers begin to move at EXACTLY the same time, and that all three (or two) fast-idle screws contact the fast idle cams at EXACTLY the same time.

When everything is setup correctly, the cold starting procedure is:

  1. Turn on igntion
  2. Wait for fuel pump to stop ticking
  3. Move choke lever fully up
  4. Crank

The engine should start with no more than 5-10 seconds cranking, regardless of how long it’s been sitting idle. Once started, you should be able to immediately move the choke lever down to just a bit more than the half-way position, leaving some enrichment. Cold idle should be no more than 800-1000 RPM.

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As Ray says, enrichment should begin in a synchronized fashion as you pass the midway point. I use detent clicks as my heuristic. I set the cable so that the first two clicks don’t move the enrichment lever on the middle carb. Then as it moves past the second click the middle carb starts enrichment.

From there you need to confirm the front and back are synchronous with the middle one. If not, then remove the split pin and screw the choke rod yoke in or out so that the outer carbs are synchronized. Now you know you can run it all day on fast idle without fouling the plugs, and when you push the lever all the way up (as far as it will go), it will be fully enriched.

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Old hardened jet diaphragms could also prevent full movement of the choke lever, stopping the jets from moving down, especially on a car that has been unused for a long time.

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And how is this adjusted? Perhaps I’m blind, but I never found a choke setting procedure in the manual.

Either with the carburetors on the bench, or if installed in the car, it helps to have a helper to operate the choke lever.

That way, one can sit by the carburetors and didfle with all the little adjustments, sometimes actually using a pair of pliers to bend rods, to get them all to synchronicity. It’s a royal PITA.

Stromberg’s, on the other hand are a piece of cake: you just have to adjust the cables and cable stops to get them to work in synchronicity.

Sounds delightful

As I mentioned, choke enrichment lever synchronization is handled by physically altering the lengths of the rods. The fast idle air flow is synchronized using the screws and a vacuum gauge. That is the final step. The screw tips should not even be in contact until you are ready to synchronize fast idle. I run a thin feeler gauge under them to ensure they aren’t touching and holding the butterflies open while I’m trying to tune.


For SUs, the choke synchronization is controlled by adjusting the length of the pushrods in the choke linkage under the carbs. The cable connects to the middle carb. The other two operate via the two pushrods, both of which have adjustable length. Setting the enrichment to begin at mid-choke is controlled by adjusting the clamp on the outer sleeve of the choke cable, but only AFTER all chokes are properly synchronized.

For the S-Zs, there is a separate choke cable for each carb.

Throttle synchronization is controlled by adjusting the positions of the arms on the throttle shafts that connect to the drop-links.

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Thanks, I’ve never had an issue setting the throttle sync, or fast idle setting, but had never seen anything on doing the enrichment adjustment… Will have to dig into that a bit further.

My choke cable & actuation is of course totally different being in an XK-140, I don’t have the lever with detents etc, just a knob I pull.

IIRC the manual doesn’t make mention of it. I got my cues about it from Jerry M years ago.

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Ray…By adjusting the clamp…do you mean moving the cable sleeve fwd and back? thus moving the levers?