Blow into the each hose. Or better yet, blow into the holes directly. The one that sounds like its making bubbles is the fuel supply. The other is the vent.
The fuel supply side has a "straw" attached that draws fuel from 1/4" or less off of the bottom; thus blowing air in will cause fuel to bubble. The vent has the opposite need - it purges from the top...
Good luck
P.S. Like Denny said the sizes are different the vent should be larger.
My vent is forward port corner of tank and fill and supply are aft center of tank. I also have a 72 Seafari but the logic Denny mentioned, applies to all fuel tanks
I think you already checked this, but make sure your vent is not clogged by some crap.
Post some pics if you can.