1980 Yamaha XS 400 Special wiring diagram

SOURCE: 1980 Yamaha XS 1100 Carduretor diagram

It is part of a detent mechanism for the choke. No, all 4 carbs do not have them, only 1.
Here is a link to Yamaha's website, where you can pull up a diagram of your bike's carbs. The ball is #10 and there is also a spring with it.
http://www.yamaha-motor.com/sport/parts/home.aspx
They are saying there are 2 balls and 2 springs, but I do not remember them having 2. I also don't see another #10 or #11 on the picture anywhere. You should be able to look at the rod that connects all the chokes together and see if there are 2 sets of dimples for the balls. Obviously there will be one set in carb #1, if you find another set of dimples for a detent ball, then there indeed would be 2. Also, the carb body would be drilled for the spring and ball like the #1 carb is.
Thanks
RSRBOB