1) Replace Needle – Loosen the nut two turns. Remove the old Needle and replace it with a new Needle. Set the new needle all the way back against the Stop Pin, and retighten the nut.
 
2) Check the Threading –

A) Refer to the Threading Diagram provided with the machine, and ensure that the threading is proper.
B) Make sure the thread is coming off the spools freely without getting wrapped around the Thread Stand. 

3) Check for Burrs/Sharp Edges –

A) Determine which thread is breaking, and check for any sharp edges along its path of travel. Remove any burrs encountered, and buff the part smooth.
B) Check the Loopers for burrs. Remove any burrs and buff the Looper smooth. Take care not to dull the point in the process. 

4) Decrease Tension of Thread – Determine which thread is breaking, and decrease the tension on that thread. Turning the Tension Adjustment Nut counterclockwise decreases the tension. The standard setting is considered to be lock nuts on all tensions approximately even with the ends of the Tension Studs.
 
5) Problem with the Thread –

A) The thread might be brittle from a dying process or from age. Try another cone of thread from another lot if possible.
B) The thread might be too light a size/strength for the application.  

6) Check the Looper Settings;