The vent is closed in storage to keep out moisture and keep the light ends of the fuel mix (which promote easy starting) from evaporating, and closed during transportation to keep from spilling fuel. When the motor is running however, the vent must be open so air can get in to make up for the space left by the fuel that you use. If the pressure in the tank tries to go below atmospheric, no fuel can flow. – – – 😉

Garry in Tampa