Riding Lawn Mowers

Model YTH21K46 expanding gas tank. I've had it under a year. This weekend gray smoke poured out from the front. It was gasoline, leaking out of the carb, hitting the hot exhaust. The air cleaner was full of gasoline, the filter soaked, and the gas tank had expanded like a basketball! I removed the gas cap and it shot off, with the tank returning to normal size. There is a hose from the bottom of the carb to the top of the gas tank, which I suspect is over-pressuring the system. I did notice that the gas tank was only about 1/8 full. Any suggestions?