Overlocker weirdism

So one thing about overlockers is that most of them come with a feature called differential feed. This lets you vary the ratio between the rate at which fabric is fed in and out of the area under the presser foot and needles. It is supposed to be a magical fix for tricky fabrics where seams won’t lie flat. If the seam is stretching out you make the inward feed faster than the outward feed, and if it gathers or puckers you make it slower.

This sounds great in theory and you can find loads of blog posts explaining it. It’s also what my machine’s manual suggests to do to correct stretched or gathered seams. But it never really works for me. I can see a small difference when I adjust the feed, but I often find I get gathered edges on lightweight fabric even with the differential feed down at the minimum. This sample is a case in point: three thread overlock seam finish on a lightweight fabric with the differential feed at 0.7 and everything else at the default settings. Awful.

Puckered overlocker edge

The problem seems to come from the needle thread; the loopers look fine. Reduce the needle thread tension, and suddenly all is well.

Flat overlocker edge

Does this happen to anyone else or is it just that my machine has overenthusiastic tension?