The end carbs have idle mixture screws too, but if the carbs are new or original they are capped. In terms of idle quality, float settings are critical, as is the correct power valve like CP mentionned and idle speed. IMO, you should be re-curving the distributor to get more initial and the same total again, just like CP suggests. timing needs to be set before the carb does. So I'd limit the centrifical advance first, then take a vacuum reading at idle in gear if it's an auto and get a closer power valve. Then do the carb work.