Every day, spray the shaft with penetrating oil and give the counterweight a few light taps with a hammer. Tap side-to-side rather than torquing the valve the way it is supposed to turn.
There is a bit of "end-play" in the shaft, and after a few days you should begin to see movement. After it starts to move then you might try twisting it gently. You don't want to break anything.
You could also try heating the shaft occasionally with a propane torch. The expansion of the metal might break free some of the corrosion.
