Take off the plastic cover (3 screws).
grab the shaft (covered with a rubber boot) with vise grips and unscrew the top nut (clockwise).
then with your hand see that the rod can freely move in and out (about 1 inch). If not then there is binding
and it needs to be dissasembled and cleaned.
To check that the valves aren't stripped or broken take off the exhaust pipe and look inside.
To the left and right of the main exhaust port look and see that the valves are moving (opening and closing the
port) as you move the rod in and out.
If all seems OK then put all back together except for the plastic cover. Start up the bike and blip the throttle
so that the revs sometimes (for a split second) exceed 6000 rpm. Check that the rod is moved out by the system
for each moment when the revs are high. If theres no was no difficulty previously when you moved the rod in and out
and the rod isn't moving out with high revs then probably a small part inside the cases is broke off by someone
who before did not keep the shaft from turning (with vise grips) when removing or installing the top nut.