[Yosemite Sam]

I don't think I've ever seen a cast end deck with a spring tensioner idler pulley.

I believe the idler will have a slot where you manually adjust the tension, if you didn't loosen the pulley when putting the belt on, it probably was difficult to install. The question is, how tight is the belt once it's seated in the grooves. Putting the belt on the largest pulley last will make installing the belt easier.

Remove the belt, turn the spindles and idler by hand, see how they feel and act accordingly. Might not hurt to take them apart, clean them out, inspect the parts and look at the exploded view (from the tech section) to make sure they were put together correctly.