My guess would be one of two things....The float valve either has a piece of trash or fuel gum in it,stopping it from sealing up when the fuel level reaches it's full level in the carbs float bowl....Other thought is,if it's been setting up for a year with fuel in it,and the fuel has ethanol in it,the rubber sealing portion of the float valve could have become deteriorated,causeing it to become brittle,or even fall apart..Either way,I would definately center my efforts toward that float valve.....
Hoodlum