Next time you start it and it's idling, push up on the pistons. There should be a little pin with a spring under each carb. If the engine speeds up it's rich if it slows down it's lean.
Also those pistons in the carb should move freely from top to bottom. You should be able to reach into the carb (engine off!) and push the piston up then let go and watch it drop with a "thunk". Both carbs should move the same.
The screw thing on the botton adjusts the overall mixture of the carb. It moves the nozzle (hole that the needle is in) in relation to the needle. You should be able to adjust the idle mixture with this adjustment.
Off idle, when the pistons/needles move up, it's the profile of the needle that changes the mixture.
SU tech pages
Try this link. I've had to read and reread it several times as well as other pages i googled.
edit: I turned the screws/knobs on the bottom all the way in, marked them, then turned them back out. I counted how many turns so that both carbs were the same. Mark the knobs so you can see the marks when you lean in to adjust them. Drive around... half turn in... drive some more ... full turn out... etc