Your mid range is likely rich and therefore you are correct in turning the tiny airbleed screw but which way did you turn it? On a bleeder type carb the airbleed screw works in the opposite direction of the main needle valve. In other words the main needle is screwed in to lean the main mixture. But the midrange screw is screwed out to lean the midrange.

Another factor is you may have not set the midrange screw at a reasonable starting position in the first place where did you set it.

John