Thread tension? The thread continually knots up on the back of the material. I have tried adjusting the tension tighter, but it does not seem to help. I can sew a few stitches & then it knots up again. I have re-threaded everything to make it is correct in the needle & the bobbin.

I own this little gem of a machine. It definitely sews a perfect stitch. I'm assuming you own the green 3/4 size machine as denoted by your product number. The half size blue one pictured is incorrect.

Anyway, this machine sews a perfect stitch, due to the oscillating bobbin. Most sewing machines with rotary (drop in) bobbins don't have an easily accessible bobbin tension screw, which means your fabric is usually slightly puckered. Fiddling with only the upper tension helps very little. This frustrates me to no end for sewing long curtains, clothing side seams, etc. Wrong tension causes them to hang with puckers, making them look awful. With the Hello Kitty 3/4 size Janome, there is a screw on the bobbin case which allows you to adjust for every thread diameter perfectly. Always a perfect stitch, if you take the time to adjust this screw.

To adjust the bobbin for a perfect tension, load the bobbin into the bobbin case and thread it through the guide. Holding only the thread between two or three fingers, let the bobbin case dangle below. This will be slightly difficult, because the bobbin will want to fall out of the case. Don't worry, just don't move it around too much and it will stay in long enough for this test. If the metal bobbin case slowly drops lower, unwinding thread as it goes, the thread is too loose. Tighten the screw on the side of the bobbin case a little (about a quarter turn.)

If the bobbin case seems to be dangling firmly, give the thread a gentle tug, lifting up fairly quickly. If it doesn't release a couple of inches of thread, it's too tight, loosen the screw. You know the tension is perfect when a quick, light tug of the thread releases a couple of inches of thread.