Check the curtain for pinholes - which can happen by leaving the cap off in bright sunlight. I've spent some time disassembling, cleaning and repairing Zorki/FED cameras. I've had variable results on getting the shutter speeds back to spec - some of the cameras are just too old and worn, I believe. Speeds are often about 1/2 of dial speed. They are interesting cameras, and always generate questions when I use them in public. I've also been surprised how sharp the lenses can be, considering the usually low price. I would post some photos with my Zorkis but I'm away from home.