Try changing the thread. FWs thread from right to left through the needle. Make sure the throat plate has the pin from the shuttle in position as well. I cleaned up my Mom's FW machine for Christmas last year - here are some pics - there is one of the throat plate and how the pin should go
http://www.quiltingboard.com/vintage...t-t174236.html see window #15 Also check your tension. When did you last change the needle - is the new one in the correct position - see your owner's manual.