I have the same exact setup.
Wow, thanks Paul T, it never occurred to me that the valve was one way. My dink and motor have been sitting in the sun all week. I just sent my wife down to the boat to check the gas tank, damn. I should have thought about that.
If you are tired of having gas squirt all over your hands when disconnecting the hose it can easily be fixed. Right in the middle of the cap there is a small rubber "duckbill" valve which allows air in but not out. By removing, just pull it out with a pair of long nose pliers, the tank will then vent in both directions
Not wanting to go to EPA jail, I am not recommending anyone actually do this, but it worked for me.