Try working the throttle - with the engine OFF - using the short length of cable that is exposed right where it attaches to the carburetor.
It's sometimes easier to do this with the throttle springs removed, but REMEMBER to reattach the springs before you close the hood.
You may find that the 'stiffness' is the return spring on the secondary throttle shaft offering more resistance when you push into the secondaries.
If it is a cable problem, you will be able to feel the binding in the cable. In that case, disconnect the cable at the carb attachment and check the carb alone for smooth operation.
Good luck!