If its hot out and there is no vent, air pressure builds up in the tank and puts excessive pressure on the needle and seat until gas forces its way through.
I need to get a new cap on my '50 which will actually vent (I think my "vented" cap is defective) because when I take the cap off, I get a major rush of air out of the filler neck. I never have more than a quarter of a tank of gas in it unless I am going on a long trip. I suspect that adds to the significant amount of pressure in the tank (more air, more potential for air expansion).
Carl
