For the 650, first thing you want to do is set your handle adjustment in most of the way, Give a couple turns to full in sort of space for just a little spacing. Next, at the bottom, have it threaded in most of the way (so is sticking out to the clutch linkage. This will give you max slack for your starting point.
Now, there is a big threaded nut in the centre of your cover. A giant screw essentially. You gotta spin that off. Underneath, there is a screw with a locking nut. 10mm nut. Use a valve adjust tool is the easiest way, else a socket and put a screwdriver through.
Loosen the nut. Then adjust the screw until you start to get spring resistance, back out 1/8th to 1/4 turn and lock down. Don't forget that if you are using a socket and not an adjuster to account for what the screw will turn as you lock it in.
Lastly, adjust the cable. Do this from the bottom locking nuts. spin the one on the end out and move it until it gets near tension point, and fiddle with it until you get the handle feel you are looking for. You should see the nut and plate move as you pull the handle. The bit of motion where it doesn't move is your slack, you want a little bit, the rest is the motion that releases the clutch plates.
Because any cable works entirely on tension, if this cannot get it dialed in, then the cable is wrong (from sheathe end to connection slack improper) or is something mechanical.
Edit: Adding my personal experience with my 650. It was way out of adjustment under the cover for it's pre-tension, and probably for a while. The clutch had a problem with glazing and slipping for a while because of it. For me, they had turned that nut in too much to compensate for a siezed cable. Your's probably is turned too far out so that the clutch throw isn't releasing the clutch plates. Mechanically, they are extremely simple, isn't too much that could cause your problem outside of wierd mechanical failure or clutch plates frozen together somehow...
