Not sure what type of carburetor you bike has, but it it's the type with a float bowl, my bet would be a stuck float valve. Does is continue to leak while not running? If it does then turn your petcock to the "off" position. It will take a short time for the gas to stop dripping out. If it does stop, then the valve is stuck open, in which case, you will need to take the carb. down and give it a good cleaning.. Good luck.
Mac