Apologies if you've read this already, but I've come to consider it the Bible for tuning Weber carbs:
If you haven't already, take a half hour or so and read it all the way through.
Once you're sort of dialed in, the following part is, for me, the crux of doing routine 'touch-ups' when the motor starts to drift off-song. It's probably a good idea to clean the idle jets first to make sure dirt isn't the culprit.
"...Now go to each mixture screw one at a time and slowly turn it in (by slowly, turn it about 1/8 to 1/4 turn at a time and stop and wait for the response of the engine for a few seconds) until the engine begins to slow down (you'll be leaning out a cylinder).
Now slowly back the screw back out (same method you used to screw it in) until that cylinder starts to pick up again and run smooth. This is the critical point in the mixture adjustment. Slowly open the screw a little more at a time until you reach a maximum idle speed. Don't go any further!
If you're not sure, stop there, screw it back in until the idle drops, and then start to back it out again until you feel confident you have unscrewed the minimum amount that you have to in order to get max idle speed from that screw. Move on to the next cylinder. Do this until you have done all 4 cylinders..."