| I take in and test many makes and models of sergers for repair. Often there is an issue, like with the Elna T34 just brought home, where theres not enough tension range with the dials. Nothing 'wrong' with the machine, its not designed correctly, IMHO. This shows up with rolled hems where the lower looper must be really tight so its line is right over the needle thread. Instead what happens is you get two rows, one lower looper and one the needle. Loosening the upper looper only can go so far then it looks sloppy and it still doesnt cure the problem.
Just to try an idea I made a rolled hem with my right thumb pinching the lower looper thread and with enough pressure the hem came out beautiful!
How to fix? Its ugly, but it works. I used a piece of broomstick, fastened horizontaly across the top with twist ties to the serger handle. The lower looper thread was wound around the dowel once on its way to the tensioner, providing, well, pretension. Now the dial will fine tune your stitch perfectly! Just make sure that the thread will flow smoothly and not snag on the dowel.
Dont blame me, too many years in labs left me geeky.
I found that on machines with exposed tensioners the thread can often be wrapped twice around, simple as that. The fix posted above is for machines with embedded tension discs