Flip fuel cap on Duster need to be vented?

Here is the deal. 70/71 and maybe? early 72 did not all have evap controls. NO MOPAR CAP is supposed to be vented. The vent is suposed to be provided in the tank system. If you do NOT have evap emissions, look in the 69 shop manual (download them from MyMoar) to see how the simple vent works, and you can identify it. If there is a 1/4" tube welded into the filler neck, running down through the trunk floor you have a non-evap system

If you have or have had an evap system, there is a 1/4" line running up front into the engine bay interconnected with the evap controls. This line MUST be open, even if the evap stuff is removed because that IS the tank vent

If you run a vented cap you run the risk (with a full tank) of running fuel down the fender

Link to MyMopar service manuals. You may have to take a Dodge / vs a Plymouth or vice versa

Service Manuals – MyMopar