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1966 Cooper 998.
Starts on choke OK.
When warm with choke ‘in’ it hesitates when depressing the throttle.
Pull choke ‘out’ 1/4” and it’s responsive.
It smells rich and the plugs are sooty.
What is going on 🫤

It could be rich at idle but lean under acceleration - have you checked the damper oil or made any checks / adjustments to the mixture and idle settings in the first instance? Assuming you're running original twin HS2s per factory Cooper, are they also balanced across the pair? And what sort of condition are they in? If they're worn then a rebuild may be in order, as no amount of tuning can fully rectify a worn carb which will give inconsistent fuelling.

They are 1 1/4" SUs and in good condition.
No wear in the spindles, etc.
I wound both jets up to the top, wound them both down 12 flats and then checked their depth with vernier calipers.
Do you think the condenser is faulty

if its to lean it will lead to incomplete combustion / high HCs which can be misinterpretted as too rich a mixture, assuming all other things are correct like timing at certain RPMs

Damper oil?

I don't think that you're going about this the right way 62B

How have you ascertained that there's no wear in the spindles?
Winding both jets down an equal amount id just a rough starting point. Now what.........? Now you need to separate the carbs and get each riunning exactly the same over the cycle.
Adjust the intake by listening to the intake 'hiss'
Then adjust the mixture to get it to the bunsen blue colour on each carb
Go back to check the intake volume......, then check the colour again
Get the tickover speed right and adjust each side so that the 'hiss' is the same.

Once each carb is the same (it'll never be identical of course for several reasons...), now is the time to adjust the choke cable and levers so that it operates each carb ate EXACTLY the same time and amount.

That done, do the same to the throttle levers so that the cable operates each butterfly linkage at EXACTLY the same time.

Remove colourtunes, replace plugs and take for a test drive. A lot of people slag off the old colourtune. Not me....... Been using a pair of 'em for 50 years

No colourtune, forget the above and take to rolling road or someone as competent I say

I've never used Colortune, but it sounds a good idea.
I've done the disconnection of the joining rods and the hiss listening.
I've put 20/50 engine oil in the dashpots, perhaps it's too heavy?

I'm sure that everyone of a certain age knows that a colourtune plug is a spark plug with a body made from clear ceramic material. This allows you to actually see the mixture being burned with a colour that goes from an over rich bright orange to a correct mid-range bunsen blue to, well......nothing except a blue glow for a weak mixture that is strangling the engine!

Or, as you learned on the bunsen burners at school, the burning range of a gas.

As I mentioned in the SU Q a couple of threads ago, setting the trusty and reliable SU carb - or carbs - is best taught on the bench in a classroom atmosphere. But the little SU's are, in my limited experience and opinion, as simple as it gets and as reliable as a clockwork Omega

I have a question? How long can you safely run an engine during the tuning process with a colourtune fitted? I’ve used them but always been worried they would overheat and self destruct

Good question and I don't know the answer. But being ceramic, I just use mine until the tune-up is finished, regardless. If they were liable to separate or explode, they wouldn't be allowed to sell them is my guess.

I use two. One in No1 and the other in No4 cyl. One had a hairline crack in it but it carried on for 10 years or so until I bought another, 2nd hand off a forumer a couple of years ago.

When I've finished I pour a bit of vinegar into the underside of the plug and swill it around with a small paintbrush to clear the carbon from the ceramic 'lens', rinse and that's it until next time. Hopefully in a few years time!

Get a helper and have them pull the choke knob while you watch the jet. Typically the first half of the choke adds more air by opening the throttle plate slightly and the second part adds more fuel by dropping the jet. My guess is that you are rich and opening the choke by the amount you have described is leaning the mixture.

If you pull the choke all of the way out does it make it better or worse?

What plugs are you running?

Sorry for the delay in answering.
If the first 1/4" of the choke is weakening the mixture this would make sense.
Pulling further does make it worse.
The plugs are NGK B6E S.

First up - B6ES plugs are a bit on the coil side for a 998. I’d start with some BP5ES plugs (projector tip and a little hotter). Make sure they are gapped to 25thou if you’re using a standard ignition system

I’d do a full ignition system check before you go any further

In regards to the fuelling you really need to get it on a rolling road or use a proper air fuel ratio gauge to accurately measure the air fuel ratio. You need to set the correct idle mixture evenly across both carbs, measure it under load, and how it reacts to opening the throttle quickly. This will give you certain results as to what to change - the needle, damper oil etc

hi
well years ago when i worked as a fireman on the firebrigade in sydney we had dennis jaguar fire engines and they had 4.2 jag motors with 2 big su carbs and were always running rough and we were having change overs all the time so they could go and be sorted but always came backthe same so one morning at the start of the day shift and being the motor driver for the shift and not wanting an old motor which was on its way i had a quick look at the carbs and the dampers were bone dry so i made some light oil out of engine oil and kerosene and put that in and it ran perfectly
the motor officer arrived with an old ford trader unit got in the jag motor and started it and it ran great and wanted to know what was going on so i told him he said he would tell the mechanics about that so keep the oil up to the dampers
cheers roger mcnab

Do you know whether you have the correct needles and springs fitted ?

Might be worth listing what you have checked so far, it will help diagnose the issue

Why not post a picture, it maybe that it flushes the problem out

I hope that no one objects if I throw a little aside into the topic, with great respect to ATM's reply a few matters ago.

The first 1/4" or so of choke cable movement has - OR SHOULD HAVE - absolutely no bearing on the state of the choke, the jet or the enrichment of the mixture. All that initial movement does is to open the butterfly cam a gnats knacker, sufficient to raise the tick-over speed. See below*

For example: You can use this facility to set the tickover WITHOUT altering the mixture. For example, to set the tickover sppeed to manually set the ignition against an RPM gauge.

It is only after the tickover speed has been increased, that further outward choke cable movement will operate the cam that draws the jet downwards and it is THIS part of the choke cable movement that give choke - or enriched fuel to start the cold engine. This, together with the slightly higher tickover speed* allows the cold engine to run smoothly without stalling due to being flooded - as it might be without the enhanced tickover speed.

Excuse me if I'm going over old ground without being over technical. Because SU's are a fantastic example of simplicity.
(After all, if we used them on our meteor engined tanks in the jungle, the desert and at 30 below temperatures, that tells me a lot)

Thanks for all your input.
I've ordered all new ignition parts from The Distributor Doctor as a starting point and then back to the carbs.
I'm going down this route as it ran reasonably well before