If you are using c&c #35 thread it is probably quilting, top stitch, or buttonhole thread. You could have alo picked up a spool of hand quilting thread, which is TERRIBLE for your machine. It has a coating on it to make it easier to hand quilt, but gums up your machine. The other, larger threads will be ok in your machines, you just need a larger needle with a larger eye, and perhaps an adjustment to top tension.