70 suburb gas tank help?

[trusty72]

ok mine had the dual tanks from the factory, but i am only using one (21 gallons in enough for me). The problem is when i try to fill it, gas comes back out almost as fast as i pour it in. What is hooked up is the filler neck, the vent hose that goes from tank to filler neck, and the fuel pickup. I think maybe i need to tee into the vent hose and run a line up to the charcoal canister under the hood for more venting? Anyone have any other ideas? Im about ready to drive this hoss.