Quote Originally Posted by Lamar View Post
Yep. I was afraid that would be the answer. I was hoping that since the fast speeds work fine there may be a simple fix to this, it looks very clean inside, no dust or gunk. I hesitate to do a complete CLA myself yet. I do have a 1962 Zorki 4 I got for $15 that I have been using as a training tool. The front curtain ribbons have come loose from the drum. I have it disassembled and am learning from it but I hesitate to screw around with a mostly working camera just yet. What specific points affect the curtain travel speed and why does it only affect it at slow speeds and not fast speeds? The return spring is the driver and it is tensioned the same regardless. Once the curtain is released to return, it should travel at the same speed from what I can deduce from my "parts" Zorki.
The curtain speed is a fixed quantity, it is the same for all shutter speeds. Unless you have instruments to measure curtain travel time and shutter speed (effectively, slit width) you can't really tell what's going on, except by making test exposure - a crude method at best. If you can actually see that the curtain is sluggish, it's way out of whack. What may effect it at slow speeds (large slit width) would be poor lubrication in the drum and roller (under the shutter speed dial) and associated timing mechanisms.

These shutters are pretty simple once you understand the theory and get used to working with smallish mechanisms. Keep practicing, you'll be fabricating your own curtains in no time!