KenmoreRulesAll,
I love your machines. You have a good eye. If you're getting them for $9.99 each, I am so so jealous. That Singer 237 is reputed to have a great satin stitch (don't have one myself) and is reportedly the last all-metal Singer.
Jake,
I would check threading diagram and make sure the needle is inserted correctly first. The flat side of the needle should be on the left as you insert it and it will be threaded from right to left.
If your machine has an external motor with a belt, it's a 15-90. If it has a potted motor on the back, it's a 15-91. They use the same manual.
You can find a user manual here:
http://www.ismacs.net/singer_sewing_...uals/15-91.pdf
And the Adjuster's Manual is here:
http://parts.singerco.com/IPinstManuals/15-91.pdf
Good luck with your machine. It's probably something simple. You can start a new thread if you have more issues. Welcome to the board. It's a pretty nice group here.
John