Tuning CZ200 power fluctuations

[Poopshoot55]

Hello all, I have been shooting my cz200 and noticed that after about 15 shots, 1 shot will drop 50 to 70 fps, then get 3 good shots, and then another 1 shot drop, and continue on like this. I changed orings, cleaned the barrel, and even put a new air tube on, thinking the valve was bad. The gun is shooting at 12 fpe, never changed the power because it’s such a nice shooting gun. Any cz owners experience this problem or any input I would appreciate. Thanks Rick

 

[nervoustrig]

Some of the potential causes of erratic velocity swings for PCPs in general:

1. Hammer friction is probably the most common. Does the hammer move freely about its full travel? Are both the hammer and its runway clean and smooth? Not necessarily polished but free of burrs and glaring machining marks?
2. How is the hammer lubricated? Oil or grease can cause fluctuations due to their viscosity, or in some cases contribute to an air cushion (or vacuum) effect as the hammer flies forward. Try a dry lube burnished onto the parts (graphite or molybdenum disulfide or tungsten disulfide).
3. Another potential source of hammer friction is sometimes the trigger group. I don’t remember if the CZ200 trigger group has an overtravel adjuster, but if it does it wouldn’t be a bad idea to back it off to rule out the possibility.
4. A breech seal or bolt O-ring that is marginally sealing. That could be because the seal is nicked or worn or cracked. Such swings can occur long before it becomes obvious enough to detect as a puff of air in your face. Try replacing it for good measure. While replacing it, confirm there is a slight interference fit when closing the bolt. If something is out of tolerance, the O-ring may not be getting the little smidge of compression it needs to seal reliably. FWIW I ran into this problem on an S410 a while back and had to sleeve the bolt to bring up its OD so it would work properly.

 

[Poopshoot55]

I did replace the oring on the bolt probe and it does snug up going into the breech. Will look into the trigger and hammer. Thanks for the ideas. Rick