Third thread--is that the upper looper? Remove all the threads from the machine. Clip them below the thread tree, RAISE the presser foot, and then pull all 4 threads down through the needle. (Never Pull Up!!)
ALWAYS RAISE the presser foot.
Rethread beginning with the Upper Looper, then Lower Looper, then the needles. Be sure to ALWAYS thread in this order!! (On most newer machines, when threading the Lower Looper, always finish with the Lower Looper "draped" over the top of the Upper Looper.)
If a thread breaks, remove the threads and start over from the beginning as trying to fish a thread through the existing threads very SELDOM works.
Verify each thread path is correct.
When threading each thread and to insure the threads are seated in the tensioners, give each thread an extra little tug when going through the tension disk.
Be sure to oil your machine frequently to keep it operating smoothly!
Make It Handmade Threading Your Serger or Overlocker
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