Here's an interesting thing I learned when I brought Grandma's sewing machine home. My Mom had said years ago that she couldn't get it to work, because there was something wrong with the tension, and no matter what she did, she couldn't get it to sew properly. So it spent years in her garage. I decided last fall to take it on, and brought it home.
Well well well, thank goodness for Google. I found that it is a common issue with Singers, and that, believe it or not, it's caused by the needle being put in backwards! I checked the machine, and sure enough, turned the needle around and had a brand new machine! With my old Brother machine, there's no doubt about which way to face the needle when you're putting in in, because it has such an obvious flat back on it. With the Singer, it's not so obvious, and you really have to watch it.
When my mother-in-law moved out of her home, her family threw her machine away because of the same problem. They couldn't get it to work either. I guess they should have googled!