Nothing is more frustrating than thread constantly breaking! A number of things can cause that, from a problem with the thread, a problem with the needle, or a problem with the tension, a problem with timing. First, make sure your machine is very clean - no dust bunnies under the throat plate! Make sure it's threaded properly. Also make sure the bobbin is wound properly. If you suspect the thread is old or deteriorated from sun or age, try a new thread. Make sure your needle is correct for the size of thread you are using, and that the needle is not damaged, dull, or rough. Another thing you can check, which worked like a charm for me a while back: the throat plate hole can become roughened, probably from needles breaking or going over pins. A friend taught me to use a narrow piece of emery cloth to polish up that needle hole - particularly if it is wide for zigzagging. (This wouldn't work for round hole for only up-and-down movement of the needle.) If you do not have emery cloth available, you can whittle down a common cardboard fingernail file to the appropriate size, and polish out any rough spots in the metal with that. Wish I could draw you a picture. Hope this makes sense.
