[Spits] Fuel tank not breathing "out"

[Ryan]

So a few weeks ago, I was filling up the gas tank manually(ie not at a 
gas pump). With a funnel in, I was going slowly & taking a pause every 
quart or so. Every 3-4 minutes it "gushed back up" like a geyser.

I'm starting to think this is also why I was having idling & sputtering 
problems when I was trying to drive on a super-hot & humid weekend about 
a week ago.

So how can one check to see if the fuel tank is breathing "out" 
properly, ie air going in when fuel is going out? This fuel tank is new, 
but not sure about if the vapor collection canister is doing its job. 
This is a '79 with the 2 carbon canisters.