SU carb

I am running twin SUs on mine. There is no ex@ct position for what I think you are calling the idle mixture screw. Usually they are about 2 turns out from fully screwed in. However it depends on many thinks such as wear and tear on the carb, the jetting, the slide, etc. I usually set mine so that the plug is a dark brown (not black) and I am getting a quick pick up if you snap the throttle open

If you are lucky enough to have the older type of SU you'll find a little pin/lifter under the dash pot rim. Push it up slowly and if the mixture is rich the motor will rev, if it's weak it'll stall and if it's okay it'll rev then slow or stall. With the newer type you could use a colour tune plug or try and lift the dash pot piston with a small screwdriver.

SU carbs have 1000s of needles and 3 dash pot springs from memory yellow, red and blue. Blue being the standard, red rich. Ensure you have a new or good jet, the correct unworn needle and spring. 20/50 oil in the dash pot damper works well. Thinner oil reduces the pick response. 

And, the older SU will need the jet centred around the needle reducing wear, while the newer SU has a sprung needle that rubs up and down on the jet causing wear.

Great carbs though.

Dave