Thread knotting like crazy!/Looking for an old Kenmore manual

So, I'm sewing along and suddenly I have a huge knot of thread on the underside of my fabric. Keeps happening, so I do a little investigating and it looks like my top thread is getting looped around a part of the bobbin casing - not the case that you put the bobbin in, but the casing that the bobbin case goes in. Clear as mud? Well, I'm stumped. It's not my needle because I just put a new one in before I started this project. I don't think my thread is the culprit- I've been using the same spool for other things and haven't had any problems (those were the two suggestions I found on the internet). The only thing I can imagine is that by changing the needle position (which I did a couple of times on this project) has messed something up. Anyone offer any words of advise? Oh, my machine is a very old Kenmore model. I don't have an owner's manual because I can't find one anywhere - not even at the smithsonian (did you know they have sewing machine manuals?) Anyone else have an old Kenmore?

my old janome was doing this and it turns out it was because I'd put the bobbin into the bobbin case the wrong way round so the tension wasn't right. It took me ages to figure this out because I was looking for something way more complicated.
Have you searched online for your manual? I got my janome one online even though the machine is nearly 40!
Hope this helps.
Alison