First describe exactly what's going on with that bike..
Does it start up ok (with the choke closed)? But then, when accelerating from a stop, does it go real slow and sputter and/or die unless you keep the choke closed. How fast does the bike go if you do leave the choke closed?
Closing the choke makes the mix richer than it would be with the choke wide open.. less air is allowed in to mix with available fuel.
So, if fuel is restricted, closing the choke forces the carb to use less air in the mixture.. And this somewhat corrects the mixture's fuel:air ratio.
At lower engine speeds a rich (lots of fuel compared to the amount of air) mixture is needed to run.. at higher RPM the carburetor (normally) leans the mix automatically.