I've had this problem once before and it is easily dealt with. I was riding to work and noticed the engine started running rough. Quickly established that the left cylinder was not igniting (healthy spark from plug meaning fuel starvation or flooding). Problem? - carb float sticking! I got the bike home and drained the bowl by loosening the drain screw and draining-off and discarding the fuel which came out. Ran perfect after that. I reckon that the bike was close to running out of fuel when I filled up at the gas station the evening before. This probably lead to the float bowl level getting low which lead to the float sticking open. I'm not going to bother to clean the carbs until I absolutely have to.
Bassman