Venting for Fuel Injection Swap

67Dart273 said
No Mopar cap is "vented." When Ma started installing the emissions vapor control (carbon can etc) the caps varied and were known as "pressure vacuum". These ARE NOT operational vents. They are pressure reliefs

There are ONLY THREE vent strategies over the years, which I've already touched on The last two are really similar

1....Up until the CA cars got evap controls, which started in '70 on some cars, the vent WAS ALWAYS the TEE fitting welded into the filler tube in the trunk

2...For a few years, the "carbon can" evap system had a vapor separator can in the trunk. A vent went clear up front, the 1/4" tube which connected to the carbon can system up front THIS IS the vent

3...Somewhere around 73? The trunk vapor separator was dispensed with, and the separator was incorporated into the top of the tank, resulting in ONLY TWO tank fittings..........the fuel supply on the sender, and the 1/4" fitting to the front. THIS IS the vent

Few corrections here:
1. Through '70 (minus '70 CA) all cars had the vent line which went from the top of the filler neck down under the car. If you have this setup it's a good idea to add a small filter of some sort to the bottom of this so crap doesn't get inside it, or even just zip-tie a piece of screen to the end. This setup works but can lead to fuel smell since it vents directly to the atmosphere.

2. The '71 (and '70 CA) style vent setup was complicated. There were 3 vent lines and 1 vapor return line coming out of the driver's side of the gas tank. These went up into a vapor canister that is in the trunk leaning up next to the driver's side wheel well. Theres a 5th fitting from the vapor canister which goes to the passenger side of the engine bay via a 5/16" line. This line connected to the breather on the valve cover. The idea was that any fuel vapors would be stored in the crank case and when you start the car up it's sucked through the PCV system. In addition the external carb bowl vent was connected to a fitting on top of the fuel pump (slant 6) or a 3rd fitting on the breather (V8) to store its vapors in the crankcase as well. This design has several faults and if you ever have issues with it you'd be best served removing the vapor canister and gas tank and going with the '72+ design, unless you're trying to keep your car bone stock original.

3. The '72+ design is a big improvement from the '71 ('70 CA) design. Instead of the vapor canister and multiple vents nonsense it has a single fitting at the top-middle of the gas tank facing forward. This fitting was connected to a 1/4" line going to the passenger side of the engine bay. This was then connected to a charcoal canister. The external carb bowl vent was connected to this as well.

There's a good article written on Allpar about this:
Fixing 1970s Evaporation Control Systems

I personally have a '71 which I modified to a '72+ tank and removed the vapor canister. I did not add a charcoal canister, the vent line going to the engine bay has a small filter at the end to keep crap out, and it's not connected to the breather. I've had no fuel smell problems with this design because the vent line has such a long 'run' if you will.

I will also add that as described in the Allpar article gas caps from the factory, at least 71+ caps, are pressure-vacuum caps, not normal vented caps. I have used a few aftermarket caps and they have all been normal vented caps. I even bought a 72+ direct replacement cap off ebay in primer ready for paint and it is just a vented cap, not "pressure-vacuum". This being the case, if you are using an aftermarket vented cap, you theoretically don't need any additional venting system.