Why would you sew zig zag on leather. I think if I were going to sew leather I would do it on something less rare like a 15 clone.
I don't know about the tension unless I had the machine on my bench. Does the manual have any info on the workings of the tension? Some times they have a drawing of the tension. Then you would have to look and compare. I would probably take the tension apart and put it back together but I do them all the time. If you aren't used to doing it you might want to get it serviced.