Float is stuck. Which is basically what everyone else has said, I guess. Most likely debris in carb. Sometimes you can tap it lightly and it will stop. Or pull er off and clean it. May be a good time to drain gas, clean carb, air filter and air box. Look everything over in the carb. Make sure jets are clean, slide is clean and snapping back completely closed and no other debris. I'm not saying slide or jets cause overflow, but debris causes issues with these parts also. Good luck!
