(edit: I realized after posting that I probably should have put this in the vintage machine shop thread - apologies for not posting in the right place!)
I have this beautiful old machine that I'm trying to get running, and I'm hoping someone might be able to give me some advice. I've cleaned up old machines before but never done any actual repairs, though I'm willing to have a go if I need to.
It's a Macy's badged Japanese deluxe zig-zag, model 139 according to the plate on the front (I have a model 139 manual, and it's just slightly different but seems to be basically the same machine -- the top plate is different though with an extra spool pin, different thread guide and an extra oil hole). It seems to have been really well maintained and looks like it had been carefully cleaned and oiled after its last use. I cleaned it but there wasn't really even much cleaning to do, a very little bit of lint here and there and some old gummy oil around the shuttle race area that I cleaned up, and I oiled everything.
It has two problems that I think are probably related -- the stitch length adjuster doesn't seem to actually adjust the stitch length in any meaningful way, and the fabric meanders around under the presser foot while sewing. I oiled the stitch adjuster mechanism, and I watched while turning the dial and it seems to move freely. When I turn the adjuster dial, the feed dogs shift position. But with the dial set on 1 I'm getting so many stitches to the inch that I lost count, and with it set on 5 I'm getting somewhere around 16-18 stitches per inch.
I made sure the machine was set to sew and not darn with the buttons on the bottom right, and I've messed around with the pressure adjustment cap up top. It was initially set halfway down. I pushed it all the way down (I'm testing on a medium-light fabric) and that helped some, but not much. I'm getting a somewhat longer stitch length, but the fabric still moves pretty freely from side to side under the presser foot. The machine is really clean, but I double checked around the feed dogs and there's no lint. When I press the darn button, the feed dogs drop down about level with the throat plate, and when I press sew they come back up (this isn't a feature I've ever used much on any machine, so I'm not sure how dramatic a change it's supposed to be, but they don't come up super far). They don't feel worn down to where that would be the issue, I don't think. I'm out of ideas and have struck out searching for what I might try next.
Any advice would be much appreciated! Below are some photos of the machine: