Honda NT650 Service manual hints, section 4, Fuel system, carb draining
When you uncrew the float bowl drain screw, fuel will come out of the float
bowl drain pipe. Make sure the petcock is closed when you drain the float
bowls or you'll end up draining the entire tank, and that you have a hose
on the drain tube or you'll cover the engine with fuel.
Click on the light bit of the image for a closeup.