Join Date: Mar 2002
Location: California Central Coast
On a hot day fuel vapor will fill the vent line. Then when the bike is parked in a cool spot, the vapor will condense, sometimes enough to drip.
Also, the fuel overflow line down is down there somewhere, too. With the gas cap open, you'll notice a hole external to the seal made by the cap. If you overfill, liquid fuel will spill into that hole and the overflow tube, from which it will drip under the bike.