lots of info in the vintage sewing section:
http://www.quiltingboard.com/vintage...s-t170748.html
I have found that the older machines and those plastic wonders seem to have a lot in common - user errors. That post is old but if you cut and paste some of the links will come up. There is other good info around, too.
User error: someone bought a couple vintage machines and left their 'old' plastic wonders with me. The only thing wrong with both plastic wonders was they both had the needles in backwards. Be sure to check to see what is really wrong with the machine. Someone else brought in a machine that he thought had a messed up tension. The pressure knob was set wrong and the feed dogs were slightly dropped. The bobbin tension was set wrong for the thread. If the tension isn't happy the machine isn't happy.