I'm not sure what little screw you're referring to, but if the needle moves when you turn the handwheel manually, then all you need to do is tighten the belt a little. Don't make the belt too tight or it will stress the motor, and FWs have pretty wimpy motors to begin with. One trick you might try is to lightly sand the belt on the inside edge that comes in contact with the drive pulley. It must be slipping.