My husband and I worked for months trying to solve this problem and FINALLY we did it. I even had a YLI varigated thread in the top and Coats and Clark varigated in the bobbin. The machine Janome 1600P DB makes a beautiful stitch. This is a long posting and if you have questions I'll be happy to help if I can. [email protected]
1) I give credit to piecemealquilts.wordpress.com for their recommendations.
Most of which I list below.
2) First of all make sure that the frame is level in all directions, side-to-side, top to bottom. That includes the table that it is setting on.
3) Bobbin tension: This is much looser than you normally have it set on. The bobbin should fall steadily to the floor. Read your machine manual to learn how to loosen the tension. Be sure to go in small increments. Don't be afraid. It also helps to use a different color thread in the bobbin than the top to see the problem. Is the bobbin properly inserted into the bobbin case. Is the bobbin case properly inserted into the machine?
4)Use the correct needle for your machine.
5) Use at least a 14 and a 16 or 18 is even better. Size 20 for specialty threads if you can get a size that large.
6) Make sure that the needle is inserted properly. My machine doesn't have a flat side so proper insertion is challenging. If you can find a straight pin small enough to fit in the eye do so as this helps to determine if the needle is in at the correct angle. The eye of my needle is left to right so I use a sturdy piece of thread, some spray starch on the thread helps it to stay straight. This makes it much easier to determine if the needle is in properly.
7) The pressure foot dial is set on 0
8) The stitch length is set on the longest stitch length. I know that you actually determine the stitch length and some people tell you to have it set on 0, but I found this to work.
9)Threading: Make sure you have the machine threaded correctly. Inoticed that when my thread was breaking that the thread in the take uplever was either out of the thread guides or had crossed each other. Also, sometimes the thread had wrapped itself around the first smallthread guide and/or the outside hole on the pretension thread guide hadwrapped around the bottom of the thread guide. When your thread breaks pull a good 12" out and then cut it off assometimes it has frayed higher up. This will help to reduce continuedbreakage and your frustration. This sounds dumb but still make sure the thread is sitting on the spool properly, that the thread spills off the spool as shown in your machine manual. I found it helpful to place a felt pad underneath the spool of thread. This was just a scrap of felt with a hole cut into it. Check the retractable thread guide which is right over your spool that it is correctly positioned and not twisted around. The thread should easily pull off the spool. I found that my machine worked better if I onlythreaded the two guide holes closest to the machine on the pretension guide.
10)Thread tension: I found that my needle tension had to be much looser than what I was used to anywhere from 1 -3.
11)The quilt: Not too tight and not too loose,too tight and needles and thread break, too loose and it's difficult to move the carriage. Can you poke a finger from the bottom and grip it from the top? I found this a little too loose, but used it as a guide. The side clamps are to keep it straight and not tight.
12)The take up rail should just barely clear the bed of the machine. I found it more accurate to check this with the machine in the middle of the quilt as opposed to one of the ends. Can you fit your fingers between the quilt and the machine bed? Yes, but barely is the answer.
13) I was able to quilt in both directions, but make sure your carriage moves freely in all directions.
14) Make sure the feed dogs are down and ready for free motion.
15) Make sure the presser foot is down and ready for sewing.
16) Check both the bobbin area and the top thread tension area for stray threads. It happens.
17) My last help was to add thread lubricant, but make sure your machine allows it. I just found out that you shouldn't use this on the spool if you have plastic tension discs. Mine are metal, but still I use this sparingly. I run a couple of lines on the spool, let it sit for a few minutes then sew. I don't repeat this for at least an hour of constant sewing.
18) Strangely this also helped, every so often I pulled straight out on both the knobs of the pretension disc and the tension disc. Don't pull the knobs off. This just released the tension of the thread. I didn't adjust it, I just pulled straight out. I didn't rethread it, just released it.

19) Finally, I did discover that I had a burr on my tension disc.How to figure this out, well it works better with two people, but one can do it. Lift up the presser foot, needle is in the highest position, then manually , slowly pull the thread through the needle, listen to the tension disc. You shouldn't hear anything. Watch the tension disc, does the check spring move down? Does it stay down then bounce back up? It shouldn't. Feel the thread as it is being pulled does it have tension on it? If the answer to these questions is yes, take it to the shop and let them fix it. It literally takes 5 mins.

I hope this helps

Make sure you are using the correct needles you may have damaged the darning hook or plate as I did replace these and use the Janome HL SIZE 14 PREFERABLY needles specified for this machine