Is a stock 68 gas cap vented? I assume it is. I get a "woosh" of air into the tank when I crack the cap so maybe it is plugged. My idle has been crappy for a while now too despite some carb tuning. Maybe related?

the cap is (or was) vented. should be no "woosh"!

I had the same problem but in different form. I had gas spill upon acceleration out the cap. I added a thicker gasket to the cap and on a warm day the car would misfire, sputter and run like crap. The thick gasket prevented the venting from the cap and upon opening WHOOSH. The gas formed vapor in the tank and increased pressure which caused the motor to run crappy. Vent the tank!, either with a vented cap or like I did with a two hose connection sending unit.

I had a similar problem on my '67 that manifested itself in a different way.

The car would run fine except it started to flatten out under heavy throttle above 4000 rpm or so. I teed in a fuel pressure guage at the carb and saw a pressure drop when it would flatten out, so I figured the fuel pump was going south and replaced it along with the filter. Same problem. I inspected and blew out all the fuel lines. Same problem. I finally traced it to the vent in the locking cap that was on the car when I bought it. The vent passage had a BB in it that, I assume, prevented fuel spillage under acceleration. Cleaning the spider webs out of the vent passage cured the problem. Must have been a really small spider. Under heavy fuel consumption it would draw enough vacuum in the tank to prevent sufficient fuel flow. I later replaced the cap with an SS style cap.

The BB was your check valve,, vacumn/pressure..

Thanks guys

A vented cap will let air into the fuel tank to allow for the reduction in fuel, but won't allow fuel and/or fumes out,..for obvious reasons. If you shake the fuel cap and it rattles, it's good (assuming the gasket is also good),..much like a PCV valve.

The "whoosh" is normal,..most noticeable on warmer days which expands the air in the tank.

I always thought this added pressure helped push gas to the pump,..though I doubt the General had this in mind.

But the 'whoosh" is nothing to worry about.