I've got one in that same family! Kenmore definitely had a kink for friction drives.
I was looking for information on the one I have (mine's a 120.71) and ran into this quote on the Threads magazine forum.
http://forums.threadsmagazine.com/ga...oebuck-kenmore) But this was the most info I could find on the aluminum-bodied friction drive Kenmores.
Quote: There were 3 models of Kenmore with the 120 source #, which told you that it was made by New Process Gear corporation, a division of Chrysler back in the middle 1950s. Sadly, they were pretty much a disaster, and most of them were junked within ten years, so don't put any money into it. The problem was that they used plastic (nylon?) for bushings, to supposedly eliminate oiling. The plastic would swell and contract with humidity or friction from running, and cause all sorts of problems. The most common was that the plastic ring within the hook assy would drag so badly that the tension would change with the speed of the machine. Your 49 was the first of the series, and was followed by the 120.71 and the 120.76. They weren't heavy enough to make a good door stop.
Bill Holman
When you get yours, take it apart-they're absolutely fascinating inside. The aluminum outside of mine comes off like a clamshell, and the main drive rod hangs underneath the arm skeleton, instead of riding along the top of it.
I do agree that they're (mine, anyway) worthless to sew with. As soon as the machine starts to get warm, the wonkies start. Before I found the above comment, I spent one afternoon, sewing, unsewing, and fiddling; repeatedly. Nothing helped, including the fact that the machine would cool down just enough while I was unsewing/fiddling that it would behave...until I sewed again.
Mine came in a white suitcase, with a box of accessories and the original receipt; if you're interested, I'll go dig it out and take pictures.