Vented gas cap
 

How do I tell if my gas cap is vented.

Re: Vented gas cap
 

This is the early style for the tank with a threaded filler neck.

Re: Vented gas cap
 

30-31 style has this type of venting.

Re: Vented gas cap
 

And, if it wasn't vented, your car wouldn't run more that a minute or two before it stalled from gas starvation. The gas leaving the tank flowing into the carb bowl would create a vacuum, and when that vacuum was strong enough the gas would stop flowing.

Re: Vented gas cap
 

Had that vented cap plug up on a drive one day and was about 20 miles from my destination. Car sputtered to a halt! Remembered what I had read about this potential issue and loosened the fuel cap and heart the tell-tale whoosh as air rushed into the tank.
Left the cap loose, drove to my destination and then the 40 miles back home with a loose cap, but no issues.
I ended up drilling a couple of extra vent holes in the cap. Never had the problem again. Chap