That could be a vacuum line that goes from around carb #2 to a fuel metering valve. It's a fuel cutoff in case the engine stops.
If you could get a picture of it and post it, it may help.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.