Dear Uwe, thank you very much for detailed answer. Yes, probably some parts was installed from different pfaff models. There was an old tensioner kit on my photo above, I installed another one now (black, on the photo below) because of the broken nut on tensioner discs, It is from two needle pfaff with some modifications and unfortunately it china made, but it works. Now I think everything is ok with those little pin in tensioner discs, I saw it and disassebled all parts of tensioner plate and spring. At first I thought that problem was in wrong needles and timing, but now I guess something wrong with hook or with another parts of machine... As you noticed there is something like snapping. Also I observed that backtack is shorter than normal stitching length. I'll capture a video tommorrow
