Better mechanics than me have already replied but it does seem like a vacuum leak might be the culprit, especially since it's backfiring. That's pretty much exactly the way my rotten squirrel condom behaved. Or, it could be the dreaded "vent tube lock" in conjunction with old gas...have you checked the vent hose for kinks or blockage? Might drain the float bowls just to see what comes out as well.
