Very good point Cletis!
If that gas tank cap isn't vented, it's like trying to pour gas out of a five gallon can with out an air hole. It comes out, but slow. With a vented cap, it will come out like out of the can with an air hole. And that's just about how far it will run with out air. With a full tank, it wouldn't run near that far because there is no room for air in the tank.
Joe
