|I was working with a serger and realized no matter how I fiddled with the tension dials, it still didn't stitch correctly.
I wrapped the thread a second time around the loops on the thread stand, which made it better.
Looking closely at the first thread guides, I saw what appeared to be foam rubber underneath them.
On my machine, these thread guides can be gently leveraged off. They come straight up. I had guessed correctly; it was a sliver of foam rubber underneath the guide, but it was dry, crumbly and worn away. I cut a new piece of foam the same size and shape to replace the old ones and pushed the thread guide back in place on top.
The performance and tension is very much improved.