The 69 has the gas tank vented through a nipple braised to the neck of the fill pipe and then you usea non vented cap . Because of where the gas fill neck is if you used a vented cap every time you accelerated gas will spew out the vented cap.
JamesM has it correct for venting. As Harold states, "vapor return", is another way to call he return line because tha's what the return is there for. You will never get vapor lock again. I would recommend putting it in no matter what the size engine you have in it. If you ever upgrade to 4bbl or bigger engine it's there ready for you.
x3 Do not put vented cap on 69 tank.
Engine Test Stand Playlist: https://www.youtube.com/playlist?list=PLwoxyUwptUcdqEb-o2ArqyiUaHW0G_C88 restoring my 1968 Firebird 400 HO convertible (Firedawg) 1965 Pontiac Catalina Safari Wagon 389 TriPower (Catwagon) 1999 JD AWS LX Lawn tractor 17hp (my daily driver) 2006 Sequoia 2017 Murano (wife's car) 202? Electric car 203? 68 Firebird /w electric engine 2007 Bayliner 175 runabout /w 3.0L Mercuiser__________________________________________________________
