Fuel Vaporization Using Eddy Carb -- In-Line Fuel Pump Question

Something to keep in mind as well. "Vented" gas caps are not just open to the atmosphere all the time. They have a pressure sensitive valve in them. When pressure in the tank exceeds that of the valve, the valve opens and releases said pressure. Technically, older vehicles had what was referred to as a "pressure vacuum" system, quite often that was even written on the fuel cap itself. So it is perfectly normal for the system to have both pressure and vacuum on it at different times.