I have a Tohatsu 6HP 4 stroke with an external tank. When disconnecting the fuel line at the tank I get sprayed with fuel which then runs into the bottom of the boat. "open the vent" you say. I did. It appears the vent only vents incoming air to the tank. After the tank sits in the hot sun with the vent open I get a large whoosh when loosening the filler cap. I don't know if there is a pressure limit before it vents "out" or if the tank would eventually burst?. I am going to see if I can retrofit the cap so it will vent in both directions.
End result is that like the portable gas cans sold in California, I spill more gas than I ever did before.
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.