I believe that bolt is used to guide the barrel in and out so that the idle needle valve blocks off the fuel flow so that there is not too much fuel for idle. When this screw is in place the barrel will move in and out when it rotates. It may run wide open, but will die or idle bad hen you chop the throttle. It is not common for this screw to fall out from vibration. Suggest a drop of lock tite when you install it.