Assuming you have a cannister. G3 is incorrect. These days you cannot overfill a marine fuel tank. Read on...................
Follow the 5/8" line off your tank if it is easier from that end or the cannister will typically be located pretty close to the gasoline fill port.
There will be three lines attached to your fill port assembly. Two 5/8" lines and one 1/4" line. Plus the large tank fill line. One of the large lines goes from the tank vent valve (yes there is a valve in the tank now) to the cannister then from the cannister down to an atmospheric vent. Second goes from the cannister to the fill valve. The 1/4" line coming from the tank valve goes to a lower point on the fill valve assembly and apparently is there simply to shut off the fill nozzle before the tank gets completely full. Yep, I said that correctly. That is what the valve in the tank is. It is a float valve that will allow fuel back up the small line about 2" before the tank gets completely full. That fuel will hit the sensor on the fill nozzle and shut it off. If the (2) 5/8" lines are reversed on the valve assembly you would know it right off the bat as gas will blow back at you when filling every time not just the first time. There is a valve somewhere on the fill assembly either in betweeen the two 5/8" lines that attach or between the valve and the first 5/8" line. Enjoy your new EPA mandated fuel system!
Oh, BTW - Good possibility the o-ring is missing from the gas cap assembly, check that first.
