I've mentioned a couple times that I have refurbished a rusted up Singer 99K. Put a hank crank on it and made it go.
But from the git go I've had fluctuating tension problems. Even though I cleaned the top and bobbin tensions, and polished as much of the metal as I could there was still some burrs and pits left behind where I couldn't get to them. Till yesterday.
The top tension discs were polished on a flat ceramic plate after being cleaned.
The bobbin case was cleaned and also polished where I could.
The hook, bobbin carrier latch and thread take up lever were replaced, so they were new and smooth.
But that left the top thread guide, the thread check spring, the hook on the presser foot shaft, the guide on the face plate and the bottom thread guide.
All of those must have had rust pits or burrs on the plating. As I was sewing the surfaces were getting covered by fuzz worn off of the thread as it passed through and around the guides.
So when my wife found those spools of Button Hole and Carpet thread an idea formed.
What I did was to cut off a piece of the heavy thread, coat it with polishing rouge from my Dremel tool kit, then use the coated thread to polish each thread guide in a back and forth motion. I did this to each one until the thread broke. Then cleaned all the rouge off and threaded the machine up.
There was a noticeable reduction in effort required to pull the thread through the guides and when sewing there was no more fuzz from the thread on the guides. That problem was an easy fix once the idea came to me.
As I've been sewing with this machine, the thread itself has been polishing up the areas of the machine where I couldn't get to polish. Because of this I've had to adjust the tensions several times.
I guess what I'm trying to say is if you have refurbished a rusted machine, you can expect tension variations until you get all the rough spots polished out, and it doesn't hurt to expedite the polishing with some rouge. In the case of damaged machines like Rusty, patience is not only a virtue, it's necessity.