46" Murray mower drive belt

[Roof Repairer]

I replaced the drive belt on my Murray 46" rider today and the belt is still loose. I checked the PTO handle to make sure everything there was ok and set the handle in all three settings and still have aloose belt. Any info. on how to fix the loose belt?
I just found a spring on the bottom of the idler pulley that is not connected. Probably the problem. I will check it out.