Tammi, this is what I have done today:
1. Thread (top and bottom) - previously using Gutermann 100% cotton. Today changed to Coats & Clark Cotton
2. Needle - previously changed needle that came with machine to a new Dritz universal 14. Today changed to a new Dritz universal 11. Flat side has always been to the left. Needle has always been threaded right to left.
3. Bobbin Case cleaning - tried Jamie Wallen's tip of using pin under leaf spring. No lint/dirt but went ahead and read your instructions to disassemble unit. Cleaned with isopropyl alcohol and Q-Tip. No dirt came off on Q-Tip so I guess the first time I cleaned it without disassembling the unit it got clean.
4. Bobbin Case tension - used Jamie Wallen's tip of having the tension set so the bobbin will stand up in your hand but not lift up. I had previously used the "bounce test" others recommend. Tried test sews with tension changed the width of the screw slot each time.
5. Bobbin - Sits flush in bobbin case. Isn't warped (checked with digital caliper). Unfortuantely, it's the only one I have of the correct size. If anyone can recommend a reliable brand I can get at Joann's or maybe a LQS, I'll run out tomorrow and get some.
6. Upper tension - Had previously dis- and reassembled unit per instructions in user's manual. Decided once more couldn't hurt. Watched your ON machine video and followed step by step. Did the Fast tension test and got the same results as your video.
After all that, and in sew testing during adjustment of the bobbin case tension, there is no difference in the stitches. Still look like those in my original post with bobbin thread showing on top of fabric.
Miriam, I have checked for burrs on throat plate, hook and bobbin. Also checked the guides through which the thread runs. Nothing.
Prim Quilts, this one was in such better condition to start with than your poor first 301A that it should appreciate its life now and straighten up and fly/stitch right!
Cecilia, you have no idea how much I needed to hear that right now
Every time I say I'm gonna put this thing on the curb, my husband just laughs. He's a retired auto mechanic and used to fiddling with little bits and pieces that don't want to play nice together. I think he's getting quite a kick out of watching me and this machine. I've restored a 15-88 and a 128 previously with no problem. This one is not going to get the best of me. Well, it was until I read your comment but I guess I have another round or two in me.