two things you can try. first take a piece of material that preferably has been overlock and move it between the top tension discs.it is possible that a piece of cotton is stuck between tension discs.next remove the tension spring on bobbin case,CAREFULL NOT TO LOOSE SCREW,and see foreign matter is stuck between spring and case.set tension again and i am almost certain problem will be solved. good luck
johan, July 2012
