Loose & Knotting underneath then jams

This means either your upper tension is too loose, or there is scarring in the hook/bobbin area that is catching the thread and not allowing the thread to escape properly.

First, make sure that the machine is threaded correctly ( I am an experienced sewer and sometimes misthread). Or check that the needle is the correct size for the material being used. and check that your sewing thread is the correct thread and that it is not an old thread. ( Thread can deteriorate). Also make sure the Race is clean and free of fluff, dust of bits of thread. Hope this helps.Happy Sewing.