Originally Posted by
sewbeadit
Okay, so it sounds like my thread may be too big. It is the same kind of stuff that MaryTG is using but mine might be bigger, the label isn't on the spool.
Before I bought that thread, I did a bit of reading and found several places that told me that the heaviest thread most non-industrial sewing machines can handle is the Tex 70 (a standard where I believe a Tex 70 means that 1000 meters of that thread weighs 70 g; the higher the Tex number, the heavier the thread). It did take cranking my upper tension to the max to make it work well and consistently. I thought it was too tight and would never sew without puckering, but it looks good and does not seem to struggle. When I had the tension set where I do with quilting thread, I had large loops such as the ones you describe--I think the thread needs to be very taut in order to not spring out into the wrong shape to catch the bobbin.
ETA: both Threadart.com and Wawak.com have Tex 70 nylon upholstery thread. I got mine from Wawak.