More information arrives from quilters who are dealing with new, electronic machines. Mercy recently explained her tweaking with the Bernina830 and silk thread, and she reports now that if the stitch length is set at "0," the thread tension on top gets very tight.
I have noticed on some Bernina 440's that when the BSR is used, the tension gets tighter than if a normal free motion foot is used, but of course can be adjusted down so a proper stitch is achieved.
Many things can affect the thread tension on top, even the humidity or color of thread. Dark threads may need a slightly different top tension than ecru, in the exact same thread.
In my first book, Guide to Machine Quilting, I suggested setting the stitch length to zero if you drop the feed dogs and free motion quilt. This was back in the Dark Ages when I used a mechanical machine, no brains, no sensors. Now I don't even think about doing that because my machine knows if I lower the feed dogs I don't NEED stitch length and allows them to rest comfortably in the basement, not moving at all. So I never touch that stitch length dial when the dogs are down.
On the Bernina 830 Mercy found that if she did use the zero stitch length the top tension became way too tight. If you own this machine, don't change the stitch length at all when the dogs are lowered.
All things are interconnected. Be aware.
Hope you are not slaving away too hard here in the US where we have our big Thanksgiving holiday on Thursday. I am hoping to get in an hour or so of quilting today on some ideas I've had before I forget them, and take a break from cleaning and polishing, shopping for food, cooking.
Keep quilting! Your work gets better every day.