The filler tube seal on the tank is bad.They usually dry rot and start tearing easily.Especially if the filler tube isn't tightened near the gas cap.A new seal shouldn't be more than $15.You'll need to drop the tank to remove the filler tube.When you put the new seal in,just dab a little dielectric grease around the edges of the seal.This helps it fit in place more easily.Oh,and make sure you don't have a lot of gas in your tank when/if you do this.A heavy tank is NOT fun to work with.