Carburetors , Where does this hose go? & Runs Rough

I bought this 1980 CB750 a week ago and it hadn't run in 5 years. This is my first time owning a bike so I am not too knowledgeable about them. I decided to remove the carburetors. I cleaned them up, including the jets. I put everything back together but can't seem to figure out where this hose (black rubber hose) goes. The clear one is hooked up to the gas tank.
Can anyone guide me in the right direction?

Also, when I got it to start up, it ran really rough and couldn't hold idle. When it finally did, it would bog and die under load. I'm guessing the carbs need to be synced. When the bike is running, it also leaks fuel out of the bottom (overflow, I believe). Any input?

Looks like drain or breather? It doesn't have a vacuum tap so isn't vacuum hose. Probably connects to air-box

okay thanks, I'll look into that!

It connects to the carb vent between #2 and #3 and runs over the top of the airbox to atmosphere.