Check to see if your machine works as intended before attempting repairs: https://www.youtube.com/channel/UCCTWl_z... There are approximately 350 videos of old sewing machines at work on this channel showing stitch types and performance when sewing denim, sailcloth and other materials

Many perfectly good sewing machines end up as unusable or are scrapped purely as a result of owner attempts at “repair” that are unnecessary.

Check out the instruction manuals below to see if your make and model are included