Originally Posted by Mickey2

Tangle? Something needs sorting out. It should stitch trouble free in all stitch length settings, the only thing you need to be careful of is limiting amount of stitches at zero. I would take a second look at each link from the stitch length lever to the feed dog mechanism. If one is sluggish or frozen the machine will run and stitch but it can interfere with stitch length and feed. The next thing to check would be threading and make sure there's a good match of needle size and thread weight to the fabric. Don't forget to work on getting the tension right on both bobbin case and upper tension mechanism. Double check the bobbin case too, new repacement parts are known to be of varying quality. A good 15 case should be relatively easy too find. A 15-91 tends to be a keeper for many because of the potted motor and CB hook and race, there are a few old straight stitchers well worth a second look though.