My Bebarfald Bluebird VS sewing machine has a perfect lockstitch!

To begin with the underside of my sewing was the classic bird’s nest:

Problems with the underside of your stitching indicate that there may be one or more of the possible problems:

• Machine is not threaded correctly
• Insufficient upper thread tension
• Poor thread quality

It turned out that I had threaded the machine wrongly. Looking at the pictures in my Bebarfald manual closely, I found that my machine actually differs, and there is an extra slot in the face plate. So I took a guess, re-threaded, and my machine produced a much better stitch.

For many machines, if you’ve threaded the machine correctly with good quality thread, and with appropriate upper thread tension, the resulting stitch will look nice both above and below. With the exception of vibrating shuttle machines, it is seldom necessary to venture into the dark realms of lower thread tension adjustment. If you need to do it, Alex Askaroff has a really detailed discussion of what to do. 

Of course, the Bluebird’s stitch was showing an intractable looseness on the upper side of the stitch which requires increasing tension in the lower thread. Fortunately, lower tension adjustment in vibrating shuttle machines is a routine matter. There is a small screw at the pointy end of the shuttle that you turn a little to the right to increase tension, and to the left to decrease.

My shuttle appeared to apply no tension to the thread at all. Turning the little screw made no difference. Shuttles do wear out, and I was wondering if I would have to trial a Singer replacement when it dawned on me that my shuttle probably needed taking apart and cleaning. I took mine apart and it was not only grubby and a little rusty, there was a giant gob of lint trapped in the pointy end. I removed the lint with a skewer, and cleaned and lightly oiled the three shuttle parts (spring screw and shuttle). The result: I can adjust the lower tension to achieve a balanced lock stitch. And now to practise sewing straight on a treadle machine without stalling.