Take 3 minutes to watch this quick serging video where I explain the 2 things I learned (while selling sergers at my shop years ago) that will help you IMMENSELY master your serger threading.
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To Summarize:
1, The needle thread(s) MUST come straight back from the eye of the needle. They canNOT be wrapped around the Lower Looper.
2. The Upper Looper thread must lay further back, with the Lower Looper in front of it, closer to you. I like to lay the Lower Looper thread in the bend of the upper looper.