You don't mention what year Wing you have. If it's a GL1500, there is a hose that attaches to the gas cap (at least there was one on my 98). That is for expansion of the gas and runs out under the bike. Is that hose connected or even there. Also, you should not fill the tank beyond the fill neck because this may also cause gas to escape. The tank is a pressurized system, but the gasket around the cap may also be broken or not sealing properly.