Kiev-60 flash sync tuning
I'm new here. I had repaired and tuned 2 old soviet MF cameras so far myself - Kiev-6C and Kiev-60. The cameras I got were in a pretty bad state but mechanics were working (more or less). Using the third "donor" Kiev-6C which was beyond repair I tuned the other two - flocked interiors, adjusted the mirror, focus screen, replaced all foams/seals, re-oiled mechanism (where it needed it), tuned fast shutter and curtains speeds and re-skinned the cameras. They both working fine and I run quite a few films through both apart from one tiny thing. The Kiev-60 has erratically working flash sync on 1/30 sec exposures - sometimes it fires the flash and sometimes it does not. The slower speeds work fine and fire it 100% so it's not a matter of cleaning mechanical contacts. So far I was unable to decipher how (or what part) triggers it and there seems to be very scarce info on a web about it.
Just wanted to check if by any chance someone had this experience?