I have found that my car does not idle well when the idle jets and/or needle valve(mixture) are dirty.
Have you pulled them to see if they are still clean? Make sure that you carefully screw them in while counting turns so that you can set them back where they were. I think you will find that freshly cleaned jets will make the carbs very sensitive to adjustment.
I would pull the jets and mixture screws one at a time and use carb cleaner/air compressor to clean them. It does wonders.
How long has it been since they were pulled from the car and given a good soak cleaning? When I bought my car it hardly ran; rough idle, sputtering, coughting, backfiring, it had it all. I pulled, dismantled, and cleaned both carbs and it made all the difference.
My car now has that unmistakable air cooled WWWHHHHIIIIRRRR!!!! What a lovely sound indeed.
Additional problems with rough idle could be caused by ignition wires, dist cap, rotor, etc... I hope this helps your problems.
Jamie
71 911T targa
http://www4.ncsu.edu/~jpnovak