Adam:

That "Extra" screw is a flow regulator for inverted flight. Since the carb is actually higher than the pump when upright, sometimes the engine tends to go rich when flying inverted.

Set the engine mixture adjustments with the plane held upright and level, then turn it over. If the engine runs the same upright and inverted, leave the regulator screw turned all the way in. If it goes rich back the screw out a little at a time and recheck until upright and inverted are the same. Usually no adjustment is needed.

Bill.