Air in the tank??

Hi everyone,
I got gas today and in the heat of the Souhern California sun, I forgot to put the gas cap on. I drove for a couple of minutes before someone was kind enough to let me know. I put the cap back on, drove off and all over sudden the engine started to develop this loud popping sound, like misfirings. It sounds like air pockets exploding. Also, obviously performance went to zip. I didn't know if I got a bad tank of gas or something, but I had no choice but continue driving. I got better after a while, but the engine now still sounds rough.

Could I have air in the fuel system from leaving the cap off? Does it eventually go away completely or do I need to have ot flushed or something?
I got a half tank of 91 octane gas (highest octane here), which is what I always get, same brand too (76).

I just had it rebuilt to spec 700 miles ago and just had in the shop for adjustments 2 days ago, so I'm positive it ran great before this happened.

Any ideas? I'd really appreciate any help on this! My shop won't open for a few days b/c of the holiday and I wanna drive my baby!

Thanks!!

It's a 66 C-code Coupe, 289, Holly Street Avenger carb, Edelbrock Performer intake, rebuilt auto tranny, dual exhausts.

- HotWheels66

Thanks for the help! I'll try it with the gas cap off and see. Maybe I put it on wrong or something.
It's a stock setup and the original gas cap. Do they usually have to be replaced?