How does one properly drain a motorcycle carburetor and ensure that no additional fuel drains into the fuel bowl(s)? I opened the two draincocks seen above the choke in the photo below, but even after a few minutes of leaving them open they were still leaking a lot of fuel; Is there no way to first "turn off" the supply of fuel to the carburetors, or do they simply hold a lot more fuel than I originally thought and I just need to wait longer? Below is a photo of the two carburetors on the 2005 Honda CBF600 in question:
A few minutes of fuel leaking is more than you should expect. Usually, when you drain a carburettor, if the fuel lines are full as well as the carburettor, I expect about 10-15 seconds maximum. If it's running longer than that, it's coming down from the gas tank.
Follow the fuel line to the gas tank (as best as you can). Are there any valves that you can shut? If there aren't, you can (as I have done on the occasion) take the fuel intake off the carburetor and plug it. Optionally, depending on your exact configuration, you can hold the end of the hose higher than fuel level in the tank. Then drain your carburetor.
Incidentally, you can also use this as a test to see if fuel is coming down the pipe to your carburettor. If the fuel intake line leaks continuously when you unplug it from the carb, you know that the connection to your fuel tank is direct. If it starts draining and the flow slows down and stops, you know that the fuel lines just held a lot of fuel, and the ebb in the flow was just slow in coming.
You have floatbowl drain screws
They are the two brass colored flat head screws on the side of the bottom of the float bowl.
They are located here
Both of screws are within the red oval. Turn them anti-clockwise to loosen them. Fuel will come out of the spigot below the screws. You can attach a hose to the spigot to drain the fuel into a container rather than all over your engine cases.