My son has a Toro zero turn with a Kohler CH25 twin. Starting late this summer, it was slow to return to idle. Now it revs up too fast and is slow to return to idle. We suspect that the carb linkage is not moving easy enough. Everything seems to be free and easy to move until you get to the carb. Should the valve which is moving in the carb be fairly easy to move? Any other suggestions of what could be causing this? Thank you.