If the mixture screws have little or no effect it is likely because you have advanced the throttle plate past the first progression hole.. There are three different types of Weber needle valves and they should not be changed to the non original type. Gordon Raymond knows the details. The correct needles are usually about 4 turns out from fully closed.
Your symptoms could be retarded ignition so check that. Also be aware that your emissions carbs require type 2 idle jets, say 55F17, 50F21, etc. F8, F9, F11 will not work properly in these carbs. Also the emulsion tubes should be F34 or F41. F9, F16 etc will not work properly.
76 Suzuki GT500
2011 Jaguar XKR
A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke