My repro tank (69) did not come with the vent tube in the fill neck (I added it so I could run the original vent tube though the trunk) but without that vent you will need a vented cap. Pressure and vacuum in the tank can be a problem, my old tank had a plugged vent that caused the tank to “oil can” eventually cracking the tank and leaking. Could unvented pressure in the tank push gas through a float bowl vent or somewhere else in the fuel system? Do a thorough visual. NO SMOKING!