Re: Educate me on my gas tank please
That vent screw is supposed to be loosened by turning in a CCW direction when you're running the motor, and tightens down to seal vapors in the tank when you're not using it.
If you were to leave the vent shut while running the motor, eventually the vacuum caused by the motor pumping fuel out of the tank without an air vent would overpower the fuel pump and the motor would starve for fuel.
One thing about those aftermarket tanks, if it's plastic and you're gonna leave it in a very hot place (which I wouldn't recommend, BTW), I wouldn't close the vent off completely. Pressure under a hot sun could build up and cause the tank to bulge, compromising its structural integrity. Better to move it into the shade somewhere. But normally you'd close the vent so the fuel is not subject to sucking in water vapor (which your motor will really not like!).
HTH............ed