The screw you are adjusting sounds like the mixture screw, and it appears to be doing just what it should. The only adjustment you make with the mixture screw, is getting the mixture right. I've explained how to do that previously: you get the engine running fast, and you turn the screw very slowly clockwise until the engine just begins to slow down very slightly. That is the lean limit - note where the screw slot points. Now turn the screw anticlockwise very slowly, until you notice that the engine just begins to slow down again. That is the rich limit. Set the screw halfway between the rich limit and the lean limit. The two limits should be only about 1/4 turn apart.
Setting the engine speed is done with the speed control lever on the handlebar. As I said before, I suspect that yours isn't working, because your cable clamp at the lower end of the speed control cable's outer casing may not be tightened. Does the cable outer move at the carburetor end when you move the speed control lever on the handlebar?
