Objective: Identify and correct the most common stitch faults on the Janome MC6650 so your seams are consistent enough to sell.
Most stitch faults are not machine defects — they are threading or tension symptoms you can read. First rule: when something goes wrong, STOP, raise the needle to its highest point, and do not yank the fabric. Three faults cover almost everything. (1) Loops on the underside (bottom of fabric looks messy, top looks fine) means the UPPER thread is not seated in the tension discs — this almost always happens when you threaded with the presser foot DOWN, which keeps the discs closed. Fix: raise the presser foot, fully re-thread the top, then lower it. (2) Loops or loose thread on the top side means the BOBBIN is the issue — wrong winding direction, bobbin not seated so the thread pulls to the left, or lint in the case. (3) Skipped stitches usually mean a bent, dull, or wrong-size needle, or a needle inserted not pushed fully up with the flat side to the back. A thread 'bird's nest' at the start happens when you begin sewing without holding both thread tails back and with the needle not in the fabric. On the MC6650 specifically, check that the bobbin thread is pulled up through the plate and both tails sit back under the foot before you start. Always diagnose on a doubled scrap of the same fabric, not your real piece. To undo a bad seam, slide a seam ripper under every third stitch on one side, cut, then pull the opposite thread free — never drag the ripper down the seam, which tears fabric.
Example: You sew a seam and flip it over: the top is neat but the underside has loose loops of thread lying flat. Reading the symptom, loops underneath = upper thread not in the tension discs. You raise the presser foot (opening the discs), re-thread the top from spool through the take-up lever to the needle, lower the foot, hold both tails back, and sew a test line on scrap. The underside is now flat and even — problem solved without touching the tension dial.
Try it: Take a doubled 4x6 inch fabric scrap. Deliberately create fault #1 by threading the top thread with the presser foot DOWN, then sew a 3-inch line. Observe the looping underneath. Now raise the foot, re-thread correctly, hold your tails back, and sew a second line. Compare the two undersides. Then practice unpicking the faulty line with a seam ripper using the cut-every-third-stitch method.