Originally Posted by Revenger715
Should valve let air in, to replace fuel as you use it, or let air out as fuel level drops and is replaced (in theory) with a vacuum/pressure?
Its to let air into the tank as the fuel is consumed. If the vacuum is too great the engine will stall.
The one way part is to avoid vapors from escaping although most installed tanks do not have any valves on them at all since they change temperature all the time and "breathe" in and out over the course of a day/season.
I would remove the valve unless you store the boat indoors (e.g. garage attached to your house) or the engine specifically requires it for some reason.