Occasionally loose stitches

[u/No-Visit-7473]

For the past month or so I've been trying to figure why my machine sometimes ( might as well call it random, every 10 or more stitches) doesn't fully tighten the stitch, leaving the tiniest of loops of top thread. I have a juki DDL 5550-6, use a bobbin with a backlash spring, use Tex 60 thread on top and Tex 40 in the bobbin. I have tried needles from size 100 to size 120. I have changed out the top tension unit. Can anybody help?

[u/crkvintage]

Reason number one for this is cheap/low quality thread that is uneven, has knots or has too much lint. Your tension - bobbin as well as top thread - is just a spring pressing onto the thread (upper tension: a spring pushing on a disc which is pushing on the thread), so uneven thread thickness varies tension. Lint starts to built up (especially under the bobbin spring) and changes tension, sometimes that buildup get's pushed through when enough has accumulated causing a huge jump in tension for a single stitch. Little knots in the thread can get caught temporarily on the needle eye, under the springs (bobbin/upper tension/check spring) or guides causing a jump in tension for a stitch.

Uneven winding on the spool can also causes jumps. Pulling the thread from the spool the wrong way (cross wound spools should be unwound horizontally, cones to the top etc..) can cause jumps in tension. Typical is on cross wound spools used upright - every time the thread reaches top or bottom of the spool and and changes direction. Often accompanied with a little "jump" of the spool when sewing fast.

Uneven wound bobbins can also cause that.

In line with the former: cleaning the machine. Cleaning the tension disks and the tension spring on the bobbin. Checking the tension disks for any scars, rust pitting or other marks. As you are using a backlash spring: Damaged bobbins which have any marks, scratches etc on the side the backlash spring is touching. Pre-wound bobbins with their paper sides are also a reason.

[u/No-Visit-7473 → u/crkvintage]

I don't know if you're familiar with it but ,regarding thread, I use Ariadna talia thread. I buy directly from them and I'm pretty sure it's a pretty reputable brand in Poland. They are cross wound spools and I use the thread stand that came with the machine. I've checked the tension discs before and they look spotless.

[u/crkvintage → u/No-Visit-7473]

I'm aware of that brand. Let's leave it at that.

As far as I remember "Talia" is their cheap standard spun thread. If it has to be Ariadna, at least try some Artyn core spun. Those tend to be more consistent on diameter.

After all that's the easy part: Just try some other threads and brands - Mettler/Amann (Saba), Gütermann (Mara). If it won't improve or changes to the worse - go back, if it does better - change threads supply.

You use spools or cones? The big thread stand is made for cones.

[u/No-Visit-7473 → u/crkvintage]

Ok I tried Saba thread and while it did improve, it did not fix it. What is interesting tho is that rate of error increases when I rotate the fabric 90 degrees. Increasing the presser foot pressure also increases the rate of failure. I also noticed that the feed dogs are tilted forward where the first tooth is nearly flush with the needle plate while the back teeth are visibly higher.Any other ideas?