I have a similar problem with my '66 Falcon. Except, there are no problems while driving. Its when I'm filling with gas, right after the nozzle clicks off, a second later it pukes a bunch of gas back up. My first thought: "Vent tube is blocked." Only problem is, I can't find the vent tube! I have a shop manual for this car and it claims that the fuel is vented through the fill tube. Very aggrevating to say the least. My '66 Fairlane has a vent system, very similar to the one in the image that 1bad6t posted - no problems there. But my '66 Falcon has the fill tube on the left quarter panel. Any thoughts with this one?