Can I replace the 2carb system with a 1 carb system on my 1995 Yamaha 1100 virago

SOURCE: changing carb jets

the main jets just unscrew out use a socket if you can but dont over tighten when refitting.undo the 4 screws holding the carb bowl on and the jets are straight in view.

SOURCE: Will not run without choke

have you checked for vaccuum leaks at the intake manifold? you can check by pouring water over the gasket area when/ if you get it running!

SOURCE: 85 yamaha virago xv1000 are the coils & cdi igntor box interchang

YES

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

SOURCE: how do i adjust the idle on my 1995 yamaha virago

On most bikes and four wheelers, this is done by a throttle stop. the position of the stop is usually adjusted by a screw. Remove the tank. turn throttle and see what moves by the carb/ injectors. When you return throttle to idle, the moving piece will come to rest on the throttle stop. You can now adjust its position by turning that screw.