http://www.weber-carburetors.com/Weber/adjust.htm
Follow the adjustment info and that will at least tell you where you are on the idle jet as far as lean/rich. Of course this sort of presupposes that nothing else is keeping it from running right enough to tune. If you can't get it running at least passable by adjusting the carb (smooth idle, little/no stumble) then the carb jetting is probably not the source of this problem. If I had to guess, I'd be thinking ignition troubles, points or timing. A vac leak could also be there, use the standard spray carb cleaner around the intake and carb and listen for changes in idle.