Here is a link that teaches how to fix the tension problems. It's a little long, because the author is a little chatty, but the information in the article is really good.
http://www.treadleon.net/sewingmachi...djustment.html
Dave McCallum has directions to fix the tension, also, in his book "The Featherweight 221 and I" I found it easy to do once I learned the workings of the tension dial.