As mentioned earlier. The Bing carbs all have a different start position on the mix screw. From this position the bike can be riden enough to warm up and then a final adjustment of the carbs performed. Also mentioned earlier the tune up should include a valve adjustment first, then points dwell and timing (new plugs often are helpful. Then the carbs can be adjusted.
Those carbs are the stock crabs for all R90/6 machines. The start setting for the mixture screw is One (1) full turn out. BTW this is a fuel adjustment, not an air adjustment like in many carbs. Counter clockwise is richer.