Rich at idle

could be either. Obviously you have ran the mixture screws in and out with no change so what i would do next is jack the idle up about 1500 to 200 and check the exhaust again if it seems better then I would say its air bleed if it seems just as rich then its prob jetting ..;.only way I know to get a base line on something like this
another thing check float levels as too high a float will make you over rich at an idle