It would appear that your rear carb is missing the throttle shaft return spring. I can see the spring on the front carb but nothing on the rear carb.

The carb spring that runs from the linkage to the top starter bolt is not stock as you already suspect.

My guess is that the factory rear spring broke and the PO fixed it with the long spring. My other guess is that a refurbushing of the carbs would fix the problem for good.