This bike should have the BS34 mikuni cv carbs. If you pull the carb bowl off. You'll see a hole in the mating surface of bowl. There's a brass tube that is attached to the carb body that fits into it. Down inside the hole in the float bowl is a small non-removable jet. This jet is the starter (or choke) jet. If it's clogged, your choke will not work properly. There is a hole in the wall of the bowl that connects with the bottom side of this jet. These bikes actually use fuel enrichment instead of choking the air. Check to make sure those jets are not clogged, but do not use anything that will enlarge them.
Here's an example (NOT an xs650 carb but same procedure)
You may also pull the slides with rubber diaphragms out. Holding the rubber diaphragms towards a light, pull the edges taught to check for any pinholes or cracks in the rubber. If there are cracks or pinholes the diaphragms or slides need to be replaced ( you can buy just the diaphragms seperately here http://www.jbmindustries.com/Yamaha650.html
). Also blow thru any passages to make sure there are no blockages.
I've also seen instances that the bowl vent is clogged and fuel can not be "siphoned" thru the jets.
Let us know how it works out.