I have just learned this :)
Either needle thread may be engaged for 2-thread serging.
1. Open the front cover and cutting blade cover, cut the upper looper thread just above the threading guide ( this is plate that has two holes for the threads) Raise the presser foot and pull out the clipped thread from under the foot. Remove upper thread from the machine (or you can leave the cone thread on, it doesn't matter).
2. Rotate the handwheel to bring the upper looper to its lowest position.
3. Rotate the subsidiary looper up and to the left, then slip the end into the upper looper thread eye from backside.(this sounds complicated, yes? but it is very simple : the looper has a counter part that is spring-locked to move from left to right and right to left. This is the little claw-looking part of the looper that when in the postion to serg with four threads is to the right side of the looper. It looks like a scorpion's tale. If you can't figure which part it is, use your finger to probe the looper and you'll feel it give a little. This part needs to be moved to the left - it will not slide over, but swing up and over and down, like an arch. Then, where the thread comes out of the looper is where you want the little claw-like part to hook into.)
4. Close the covers. Decrease the needel tension - this depends on what the stitch length is. Stitch length: 2-3, stitch width: 3.0 = L needle n/a, R needle 1; upper looper n/a, lower looper 5
stitch length: 2-3, stitch width: 5.0 = L needle 0, R needle n/a, upper looper n/a, lower looper 3.5
Those are for a flatlock stitch - for a 2-thread rolled edge :
Stitch length: 0.5, stitch width: M = L needle n/a, R needle 4.5, upper looper n/a, lower looper 3.5
Stitch lenght: 1.0, stitch width: M = L needle n/a, R needle 4, upper lopper n/a, lower looper 3.0
If you have any questions, e-mail me at [email protected]