So I put the previously owned Kenmore on the table and sewed some squares together. The width of the seam was too strong and it was*wonky* First I adjusted the spring loaded pressure gauge on the pressure foot. It was cranked all the way down, inhibiting the fabric from advancing under the pressure foot. That solved the tightness of the stitches and the bunching.
When I'd first set up the machine for him to use, I'd put a piece of mole foam on the throat plate to give him a quarter inch seam guide; the pressure foot is multi-purpose and creates a deep seam. So I got out my ruler and checked the distance from the needle to the mole foam. I adjusted, sewed, adjusted, sewed again, until I finally got it right. That helped a lot. But not with the half square triangles he wanted to sew! I suggested he invest in a 1/4" wide pressure foot, it would make sewing down either side of his diagonal line so much easier.
He also said the seams puckered. I found that the machine was threaded with Aureofil weight on top and Star cotton in the bobbin. I pulled them both out of the machine and laid them side by side so he could see the difference in weight. "Either will work, you just need to have the same in both the top and the bobbin." Easily solved, we wound an Aurefil bobbin. Voila! No more puckers.
You don't have to have a top of the line sewing machine to sew a nice seam. But knowing the ins and outs of how to set up a machine for nice seaming helps a lot. Sometimes we don't know what we don't know. And conversely, sometimes we forget what we do know because we know it so well it's become second nature.