I have 1970 Roadrunner with N97 (Emission), I put new gas tank but I closed the tube to Emission but I wonder about the vent so I got new gas gap but it was loose so it will breath so will that be normal with gas cap been loose?

Thanks

I purchased a vented cap. They are available on later year mopars. Also I run the vent tube that is J shaped and is mounted in a upward position towards the floor with the opening pointed down to the ground. You need proper venting or it will vapor loc. A vented or lose cap may not be enough.

A vent is needed and the gas cap is not vented. Non of the later mopars had vented caps, they were all pressure-vacuum and not vented.

It seems vented caps would allow gas to leak out?

Put your gas tank back in the car just like the factory did, it's best that way. There's nothing you could change to make it better, the factory did it right.

Agree the factory style vent is the way to go. As posted, "vented" caps are really pressure vacuum. You can still get a pretty good vacuum in the tank - enough to cause fuel starvation or vapor lock.

The only functional and currently available "vented" cap is a parts store cap with small hole drilled in it. This is not practical on a MOPAR as either acceleration or cornering will push fuel out the hole. On my old brand F PU, on the other hand the 1/16" hole works great since the fill is quite a bit higher than the tank and protected somewhat from "slosh".

If you have the California setup with a "can" in the trunk, it makes a great vent if it is intact. That's what I did. Condensed fuel vapor or "slosh" goes back to the tank. I ran the line that originally went to the engine breather into the frame rail. I believe Challenger1 ran it back to the engine as original. Either way it works. Or retrofit the "raised loop" style vent from a non emissions model.