Sorry for the delayed response, it took a couple of days to take the pictures and I lost my account info. I realize now it isn't the foot lift lever wear as such that is the most likely problem, but see for yourself.
In the middle position the foot lift selector causes the tensioner release arm to not move enough upwards to actually release the tension disks even with a rubber gasket on the release cup.
When it is moved all the way to the left (in fact further than the notch on the spring) and with the gasket I can get it to release.
But in this position the top tension is still out of whack. My hypothesis is that the excessive foot lift for my 3-4mm leather I'm using for testing pulls up the bottom thread too much. In fact, I've obtained the most consistent results so far by reducing foot lift as much as possible, keeping the tensioner permanently "closed" although even then the tension is uneven every couple of stitches (left two):
There wasn't a washer beneath the tension disks initially but we've managed to put one under there today. Unfortunately, that doesn't seem to have changed anything at all.
Lastly, I've got to admit I'm using 29x3 needles instead of the 29x4 intended for leather work as they are difficult to obtain around here. While I assume this to be not ideal, it wouldn't really cause these kinds of issues, right?
