I use Poseidon Jetstreams almost exclusively. Once they are set, they will go years without needing anything.
Now, the problem is getting them set properly.
What is important to remember is that these are unbalanced, non-adjustable, upstream second stages. What this means is that when you tune them to the bitter edge, it is CRITICAL that the IP is set properly as even a 1psi difference is enough to cause it to leak.
With the 3960 first stage as seen with most Jetstreams, the balancing is not all that great and it is inversely proportional to the supply pressure. Since it is inversely proportional, when they say you need to hook the first stage up to a high pressure supply bottle when servicing, they aren't kidding. Upstream valves need to be set at the minimum IP they will see during service.
The IP swing on the 3960 from 4000psi down to 300psi is about 20psi so it is not insignificant. I have done some field adjustments of first stages where the supply was only about 3100-3200psi, and when that first stage went on a cave filled bottle around 3800psi, the second stage was leaking as the IP was too low. I try to set the IP at 4000psi and tune the second stages at the 4000psi supply with an IP of 120-125. The IP will go up as the tank pressure goes down, so the regs won't freeflow until tank pressure is below about 200psi. When they do that, you finish an inhale and they'll give a light bubble before locking up. That's a great indicator on a stage bottle that it's time to switch and what I use when cave diving instead of checking spg's, neat little trick.
While the above is the protocol for the Xstreams and Jetstreams which use an unbalanced-upstream valve, the Cyklon is an unbalanced-downstream valve. With a downstream valve, you need to adjust the IP at the maximum IP, so the first stage IP adjustment, and second stage cracking adjustment need to be done at minimum supply pressure when using a Cyklon vs. maximum supply pressure with the Jetstream/Xstream
That initial setup is a lot more finicky than say a Scubapro G250 where you put the adjustment tool on there, turn the knob all the way out, turn until it hisses a little bit and back it off. That said, once it's done, you don't have to do anything for several years and they don't just randomly go out of adjustment
