Just finished my 650's first big winter overhaul, which included:
1) carb removal, cleaning and reassembly.
2) GAK kit + 127.5 jets + 3.5 on the idle screws + 2 #4 washers as shims on each needle.
3) valve clearance check
4) AIS removal
Already had a set of Cobra drag pipes with lollipops when I bought it. Last step was a carb sync after I made sure it was running okay.
Good news: Bike starts and runs okay.
Bad news: I cannot sync the carbs now.
Facts: The front carb is drawing something on the order of 15 lbs of vacuum and the rear's only drawing 5. Fiddling with the sync screw produces changes in the tone and quality of the idle, but no substantive change in how much vacuum either one's drawing. They simply never get close to each other. Loosening the screw (counter-clockwise) should improve the rear's vacuum and lower the front's, but it simply doesn't; I've gotten the screw to the point where it's almost falling out of the linkage, and it doesn't help. Tightening the screw (clockwise) does little more than roughen up the idle until it eventually dies and won't start until loosened a bit.
I already checked for air leaks around the filters (tightly mounted to the carb) and the boots/flange where they met the cylinders; no change in RPM.
I'm already resigned that I'll have to get shoulder-deep in the carb(s) again, probably starting with a "bench sync", but I'd really appreciate having an idea of what I should be looking for. Everything seemed A-ok when I put it back together...