I was just playing with one of these the other day. If you are describing the screw on the left of the picture that stands at a nearly 90 degree angle to the muffler mounting screw then you are adjusting the idle SPEED not the idle MIXTURE. The mixture screw is an airbleed like you said. If in fact you have been using the correct adjustment I suspect the hole is clogged. The one I have runs like a baby. I wouldn't give up on it. Take care, Cliff