Hopefully I can find a little help here from ya'll
I just bought a new singer 4411. It arrived yesterday. and I am checking it out
this thing has me confused. needle is a 18 and the thread is more than #69 [it is thick]
I have it on straight stitch. and
this is on the bottom
Can you try to find a website or other forum for troubled Singer operators? lol
Seriously though. If it is okay at the shorter stitch, but messes up at the longer stitch...
Have you messed around with the tension knob?
Is there a tension knob?
I guess you have messed with every setting.
Is that size thread recommended for using with that size needle?
My machine had a sticker with a phone number on it, I think that was the repair place where they'd tune it for me.
My machine is a regular Singer. I keep my one knob set on largest stitch most of the time, that is also the knob where you would adjust it for button holes. The other two settings include width and style of zigzags (or keep it straight) and the other one is for type of stitching. It doesn't matter what that one is set on, as long as I keep the other setting on straight.
Short of all of that, look up sewing machine repair places and see what they have to say.
I'm just gonna toss this out there, it may be something to consider.
We have that same machine, I have used it for vinyl and it did a good job with it but I had to change out the foot.
Maybe consider changing to an open toe foot. It is wider and you can see your work better, but the part that may help is the fact it has a 'tunnel' on the bottom side, so when you drop the foot it puts the proper pressure on the material so it contacts the feed dawg better..
I suspect what is happening is the stitch is so thick with that heavy thread that the foot doesn't put enough down pressure to make the material feed like it should because there is less pressure on the feed dawgs. The foot is actually being held up some due to the thickness of the stitch.
Do you find yourself having to tug on the material a little bit to make it pass along smoothly?
IF you do this I would go with a metal foot as opposed to plastic just because of the bulk that you are working with. The metal foot is a bit more in cost, but it won't snap under heavy use, IMO.
***I can't say for sure if that's your problem, I do know it helped with ours.***
Here is an example. Just need the correct foot, snap on, screw etc.
In my experience, there are several possible factors when a machine has issues. It could be the fabric (thick, thin, more), needle type, tension, and more. It is interesting that the stitch type changes when you change the stitch length. I keep a variety of needles to change out for the type of fabric, but if it wasn't bothered by the needle before you adjusted the stitch length, that really shouldn't be the problem.
My Brother machine has a knob that not only changes the stitch length, but also changes the kind of stitches when I adjust it, but it is illustrated on the knob that it will do this.
I think everything above is probably accurate also, but the first thing I thought when looking at your stitches is that your bottom tensions is tighter than your upper tension. It appears that the same thread is being used in both the needle and the bobbin which is important. The "knot" where the two threads tie should be at the base/underside of the hole made by the needle. It looks like you can see the upper thread on the underside of your work indicating you should tighten your upper tension (thread that goes through the needle.)