They usually stick in the closed position because that is the position when at rest.
To free it up get the engine really warm, anoint the bearing area of the shaft often with preferrably Kano Kroil or other penetrant and tap the shaft in all directions as well as the counterweight and try to rotate it often with pliers or tapping the cweight.The key is be patient, it didn't sieze overnight it may take persistance to free it.

Bob Taylor