fuel leak

In advance, thank you for your input. I have a 1997 Speedster that i bought last fall. I have on two occasions filled the tank to the top. When I do so it leaks fuel, I can't seem to find the leak. Its only when it is full. It leaks out gallon or so and then stops.. Anybody seen this or...

During this time of year we are having fairly wide temperature swings. When it has been cold then we warm up to 75+ fuel starts to fill the bottom of the hull and drip out the butt plug. I first thought that the fuel was leaking from the carbs BUT the fuel switch is off so it shouldn't be...