I'm 99% certain that the problem is actually under the top plate - I was given a Zorki 4 with exactly the same problem.
I forget the exact fix, but the cause of the problem (if I remember right) is the arm which is lifted and dropped into place around a central shaft by the shutter speed dial when you change the shutter speed. It's very easy for it to jump out of place on the Zorki 4, meaning the pins below the sprocket wheel (the ones CMOG is pointing at) don't engage with each other properly.
Here's the related section from Maizenburg's repair manual:
"This is one of the most common malfunctions. It usually comes up suddenly while you are shooting, although the shutter may have worked well before that.
"The exposure lever has jumped over" - this is the way that camera repairmen describe this problem. Indeed, it occurs when the exposure lever 4 arm gets on the other side of the pin 3. In this position its turning is restricted by the post that closes the flash synchronization circuit on one side and by the pin on the other side. It is impossible to set the "B" speed, and the curtains shift a little and stop if the shutter release button is pushed.
This problem is most often caused by two factors: first, the speed knob was set before the winding of the shutter; second, the pin 3 or the delay mechanism was displaced and as a result, the lever jumped over the pin 3 during the shifting of the speed knob from 1/60 sec to 1/15 sec. When the speed knob is lifted, the exposure lever that bears it is lifted too. It's arm along with the pin is next to the pin 3. Right away, the lever arm catches the pin 3 and turns along with the gear 2. Sometimes the pin shifts slightly (due to looseness or a displacement of the mechanism). Then the exposure lever arm does not set next to the pin during the switching to 1/15 sec speed, but instead, sets right on the pin, and at the next turn of the lever, it jumps over without catching the pin. Usually this problem is easy to correct. Remove the top plate 12, examine the mechanism and gently lift the edge of the plate with the gear 2.You should now reset the exposure lever. Lastly, check whether the delay mechanism fastening screws are tight."
I can't remember where I got my PDF copy of the Maizenberg repair guide from, but if you PM me your email address, I'm happy to email you a copy so you can see the diagrams you'll need.
