I wonder if you got the belt too tight? There are instructions (without pictures) for installing a treadle belt at
https://www.treadleon.net/sewingmach...eadlebelt.html.
Some points to be noted
- "The key thing is not to get it too tight."
- "the belt just tight enough to sew, but not to slip."
- stretching it before putting on the machine.
It is not a very good image and kind of a pain to get to page 14 of
https://www.scribd.com/document/1642...r-66k-Handbook but does show oiling points of a treadle. (You have to wait for ads and then click on the page or skip to see) Basically, it is any point where two pieces of metal move against the other that need oiling.