I have a Tohatsu 6HP 4 stroke, maybe the same motor? Maybe a stuck float needle? A few taps on the float bowl might free it? Easy, not too hard! The other possibility is that the external tank only vents incoming air. I discovered this on a hot day in the sun. When I opened the filler cap I got a big whoosh of outgoing air, LOTS of pressure, which may overcome the float valve?
I fixed it by removing the small rubber "duckbill" one way valve from the center of the cap. No more sprayed gas on my hands when disconnecting the hose. You might try running with the filler cap off to see if you still, get the overflow. If so, it is probably a stuck float valve.
An update on this: After removing the one way duckbill valve on the cap, when squeezing the primer bulb with the now two way vent open, I heard what appeared to be bubbles blowing inside the tank and the bulb would not "firm up".
I also noted that it was taking about 4 or 5 more pulls to get the engine started. After closing the vent the bulb would "firm up" after 4 or 5 squeezes.
So, it appears the bulb pressurizes the tank instead of actually "pumping" the fuel?
So, looking back at what I did, I think an easier solution to getting sprayed gas on your hands would be to just loosen the whole filler cap before disconnecting the fuel lines.
As mentioned in an earlier post, after sitting in the hot sun the tank built up a whole lot of pressure as the original cap only vents air into the tank, not out. If this is a concern you could just loosen the filler cap a bit to relieve the pressure instead of modifying the one way vent, like I did. Why didn't I think of that in the first place?