Quilting on a Jack F4
I also quilt on a Jack F4. I purchased my machine in May 2020. I have had no problems really with FMQ on this industrial sewing machine. I use Glide, Aurilux, and Guttenberg poly threads with no issues. I have also pieced using cotton threads with no problems. I am currently using Juki 90/14 DBx1 needles but have ordered some Groz-Beckert needles to try as the Juki needles seem to dull quickly. I tried Organ needles but they made a "clicking" noise so I quit using them. I keep the feed dogs up, set to the stitch length to 0, cover the feed dogs with a Supreme Slider. I use a ruler foot always for quilting on this machine, even if I'm FMQing without using rulers. As others have mentioned try adjusting you needle, twisting it ever so slightly left or right and making sure the scarf is to the right. Remember to thread left to right. I hope that you have solved your problems with your Jack. It is a great machine to quilt with and produces fabulous stitches with no birds nest, skipped stitches or eyelases! And the throat space is awesome!