Recently I had a frustrating day with the tension on my Singer 128K, and the only way I could tighten the bobbin tension was to fish the shuttle out of another machine and use it instead of the shuttle from the 128K.
This is the offending shuttle. I have finally had a look at it. First I unscrewed the tension spring and removed it. Here is the shuttle with the tension spring removed, and you can see that there is a bit of residual muck which had been hiding underneath. This took only a minute to remove with metal polish.
Here is the underneath of the tension spring, which also needed cleaning up.
On the right hand side there is a tiny groove visible, which is where the thread passes. The bobbin tension had been too loose. Even with the screw fully tightened, the tension spring was not holding the thread firmly enough against the shuttle.
The solution was simple. All I had to do was exert a little pressure with my fingers on that part of the tension spring to encourage it to lie closer to the shuttle: had I used too much force I would have risked breaking the metal at its thinnest points, so I hesitate to say I bent it. The change in shape is imperceptible on mere inspection. Once the shuttle was reassembled and I tried out the stitch, all I needed to do was make a tiny adjustment to the top tension and the stitch was fine.
Job done. Now the 128K has its own shuttle back in use.