There are two vents on the tank. One comes out the bottom of the tank and has a hose connected to it that exits on the left side of the bike near the bottom of the fairing. The others is a small vent in the gas cap itself. If the vents are not working the tank will develop a vacuum and fuel won't be pumped to the carbs. If opening the gas cap caused an improvement I would check the vents. The bottom one is easy, just blow into it. The cap vent might be a little more difficult.