it worked!

Tightened this screw and the pulley stabilized. What's weird is it was completely loose. Not sure how it loosened itself up. Maybe I should put some loctite on it...

I was told by someone who knows a fuck lot about machines and shit not to touch this screw for any reason. Well, I did. And used a tree branch under the motor as leverage (it's hard to adjust the tension on the belt alone because you have to hold the motor with one arm and bend around and turn the screw with the other).

Every once in a while I can hear a chirp from the belt but otherwise it's working. Think the belt may be slightly damaged from when the wheel loosened up. Fuck it. I'll replace it in the fall. In the mean time if anyone wants to come over to my house and turn the swamp cooler on and off for me so I don't have to keep climbing up and down from the roof feel free. I'd like to make a few more tweaks to the belt tension.