Another option for gathering is using Wooly Nylon or other soft stretchy thread/yarn in the lower looper.
Thread the soft yarn through the guides as usual. I did find that a drop of Frey Check on the thread-end helped stiffen the end and make it pass through holes much easier.
Initially, I tried regular serger thread in both needles. I used a 4 stitch length, 2.5 differential setting and moved the STS lever to the right. That produced a rather nice tunnel (as in heirloom stitching) but not much gathering. I removed the left needle and tried again.
My fabric is a light weight cotton. I’ve roll-hemmed the left side and finished the right with a 3-needle over lock both done on the serger before rolling over to the cover stitch. (My sewing chair has wheels. I literally roll from machine to machine and back again.)
In the pic above, you can already see the fabric strip gathering. After several short tests, I cranked up the looper tension to 8 and produced nice ruffles. However, tests are not the same as the real thing. For my project, I needed 1.5″ wide but 45″ long. Over that long distance the gathering wasn’t apparent. I’m assuming that the drag of the fabric both in front and behind the needle prevented the CS from achieving the same gathering it did on a 12″ length. My quick solution was Nancy Zieman’s old “Ease+” technique. I would drop my thumb behind the foot for a sec. Lift and repeat all 45″; all 4 strips.
This resulted in a pretty decent ruffle in very short time. In fact, I think I spent more time experimenting than I did making the “real” ruffle!
^^^^^^^^^^^WHAT’S IN THE COOKBOOK