Do you by chance have a new aftermarket gas cap? They come with a valve in them to make them ventless. You can remedy this by drilling a 1/8" hole in the plastic on the backside of the cap. This will make the cap vented just like the original.
I do not believe that your 20 year old kid was correct about the vent tube being a problem for this design of gas tank. The small tube on these tanks is for allowing air out while filling the tank with gas. It does absolutely nothing once the gas cap is on.
