The idle screw is an air bleed. Clockwise is richer. It should have no effect on the midrange. At 1/4 throttle you are already above the suction to pressure point. I never idle them that slow as I like to be into the regulator at idle. 2300 or so will do it.
My guess is that the regulator is set to rich, but I could be wrong. Try running the reg in about 1/2 turn and see if it improves.