Air in the tank??

[hotwheels66]

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

 

[5.0n66]

Does it run ok with the cap back off again?
usually problems associated with gascaps and engine running poorly is due to gascap not venting properly. This doesnt allow air to come into the tank to replace the gas that is going to the engine and a vacuum forms that the fuel pump has to work against and the engine wont get enough fuel.......as far as air getting into the fuel it's not a problem if it's a stock setup and you havent been driving on cobblestone roads at breakneck speeds(unless it's an EFI setup), b/c carb'd cars dont have a return line that can cause turbulence in the tank that creates air bubbles... and since it doesnt have that turbulence any air that is in the tank will stay in the upper part of the tank where it belongs since gas is heavier.

 

[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?

 

[Glen's 1965 5.0]

Vent check

Looking at the backside of the cap, there should be a small hole in the center. You can put a vacuum gauge on it and see if it builds a vacuum. (It shouldn’t) or the old fashion way is to use the lip method. You should be able to suck in but not blow out (Through the hole) This just doesn’t sound right I know.