Question about buying Carter BBD for my 318 (unless I should look at something else)

What does it mean if I can screw the idle screws all the way in, and the 318 still idles at 500 rpm, vac is 14 - 15 ?

This is with the china dromedary BBD clone.

With the screws out about 1 turn, rpm goes up, vac goes up, about 17-18 inches.

Either the screws are not completely blocking flow, or there is another path for gas at idle (that shouldn't exist?) ?