So I don't get what the idle mixture screw o-ring is for. I see the fuel passage going into the screw passage and how screwing the screw in will stop fuel flow into the throttle body. So is the o-ring simply to prevent fuel from leaking out the threads of the screw, or air leaking into the circuit from the screw threads?
It's just that this posting makes the o-ring sound so critical. With so much thread surface it seems very little air could get sucked in.