The machine is most likely not out of wack. I do a lot of carpets and have found every time a roll of yarn is changed tensions need adjustment. Also after doing a lot of practice it becomes obvious what needs adjusted. In my opinion sergers do not work like sewing machines,adjustment and practice is needed. Chances are the large spool of yarn he used was a rounder yarn NC yarn is slightly flatter which call for adjustment I currently use both types of yarn depends on who has best color match. I have also found different spoils of yarn feed differently which also can cause issues as well as what type of yarn it is wool, nylon or polyester

I should off taken my nc yarn to set up the tension for my yarn instead of his. But his does look round and a little more stiffer compared to the flat nc yarn...

That would have worked to get this job done but as I stated I have found tensions to change with rolls of yarn. I typically have love hate relationship with my sergers. Currently have both the small NC which is the same as yours and 2 81200 larger sergers ( I use one for back up) once set up they all run great

Who do you get your yarn from? Looking online NC looks to be the only one.

I get my yarn from carpet vendor (higher end wool for aircraft) NC and Burke Tool in Florida NC sponsors this forum plus they are great folks to deal with so I support them with tread and yarn when they have what I need.

On your small serger that looks like mine which NC yarn do you used.
Looks like the stitch lenght was to small for the NC yarn I bought. So they lengthen the stitch it looks alot better.

That does look better. May still need to play with the tension. I place reference marks on my tension knobs and right down how much I turn them in or out so I can turn back to where it looks good if I change it too much. Keeping good sharp needles in machine is important as well. I have found the thicker the yarn the larger the stitch length needs to be. I have used both the polyester and cotton. I really do not use that serger much any more as it doesn't do inside corners or circles that easy.

First time using a carpet serger Karpet king got it for $150 bucks plus parts that need replaced.
Practicing on corners.