Depends on which belt. Your machine has 2. Better to take it in and have them replaced because it's VERY easy to knock your machine out of time. Have them clean and adjust it while you are there.
try these suggestionschange the needlemake sure the thread doesn't come off the thread take up lever--this can jam the machinemake sure the bobbin is turned the right way in the bobbin case--thiscan cause problems if the bobbin is turned the wrong waymake sure the bobbin is not warped or damagedmake sure there are no burrs in the bobbin area or on the needleplate
The most common cause of this is improper threading. 1. Thread the upper thread properly with the presser foot in the UP position. 2. Make sure the bobbin is threading with the thread unspooling in the correct direction. 3. Make sure the needle and thread are suitable for the fabric in your project. Watch this video.
Make sure your needle and tread match your project. There simply isn't such a thing as a "universal needle." Also try lengthening your stitch. Heavier fabrics typically need a slightly longer stitch. Upon commencing to sew always grasp the 2 threads and slightly pull as you begin your seam. I assume your garage sale find has a manual. Be sure your bobbin is unspooling in the correct direction. If it didn't come with a manual.. http://www.singerco.com/uploads/download/430_8280_es.pdfGood luck.
