I was blessed with a very nice 15-91, and after cleaning, oiling, and tuning the machine (and refinishing the top parts of the cabinet), I sewed my first project. I'm not new to sewing (40 years of hobby), but I'm also not deeply familiar with machine issues and how to remedy them. This machine was last used as recently as 1 year ago by original owner.
Replaced original bobbin case with generic reproduction that came in a 2 pack. No other parts replaced by me. Original bobbin case would not stay in, the little flap lost it's spring/tension. Very funny finding that out when the bobbin rolled away from me under the couch mid-sew.
Sewing with two different thread types - one cone of cotton thread, and one spool of nylon thread. Both possibly 20+ years old, borrowed from my mom. Not used together.
When sewing with the cotton thread, about every foot or so, as well as after pausing to reposition fabric, top thread would get caught on the bobbin case. I'd stop, cut the thread, remove bobbin, clean, and replace, and carry on. Switched to nylon thread for a different part/color and had zero issues, sewed as smooth as butter. No nesting, snagging, or bunching.
Is this more likely to be an issue with old cotton thread, or more likely an issue with the bobbin case?
This is still way less of a headache than my plastic monstrosity of a modern heavy duty. First project was a success! McCall's 2378 vintage pattern.