Janome AirThread 2000D Professional Service Bulletin page 64

Adjusting the Tension
When serger tension is adjusted correctly, the upper and lower
looper threads meet at the edge of the fabric and both needle

threads are tight in the fabric.
When the left needle tension is loose, you will notice these loops
on the bottom of the fabric. To correct loose left needle tension,

turn the left needle tension to a higher number.
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