If the second stage is adjustable (G250, S600, X650) the tech is pretty safe is setting the orifice to allow minimum inhalation effort with perhpas 1/16 of a turn to anticipate the formation of a seating groove. In practice this results in inhalation efforts in the 1.0 to 1.1" of water range which is the minimum the diver can use anyway due to case geometry fault issues.
With a non adjustable second stage, the tech needs to be sure to fully anticipate the development of the seating groove and ensure the minimum cracking effort is enough to prevent freeflow between inhalations when the diver is in a face down positon. This usually means tuning the reg somewhere in the 1.2 to 1.4" of water range with the orfice turned in perhaps 1/12 to 1/8 of a turn past the point where any leaking stops. This conservative approach is needed as any unwanted freeflow cannot be eliminated with a diver adjustment and means the reg will come back to the shop with a usually annoyed diver. With a non adjustable and unbalanced second stage like the R190, R290, R380, R390, etc, The spring in the second stage is much stiffer and seating grooves tend to be more significant, so the regs tend to be detuned a bit more to perhaps the 1.4 to 1.6" of water range. And some of these regs will not delivert that good of performance anyway. The R190 in particular varies with some delivering excellent inhalation efforts and others doing much worse.
In some cases the orifice can be nicked and if not detected by the tech, the result is that a much higher inhalation effort results due to the excessive pressure needed to seal the damaged orifice against the seat. In other cases, the seat itself may be less than perfect with more pressure required for a good seal (This in my experience happens with about 3 to 5% of the seats in the S-wing poppet and you can usually spot the problem visually in good light.)
If the tech properly bench tests the reg, he or she will note the higher than normal inhalation effort and should then diagnose the issue - particularly on a high performance balanced second stage.
On SP's balanced second stages the design is a "Single Adjustment" design where both the spring pressure and lever height are controlled by the orifice adjustment. Good engineering and close tolerances are needed to ensure that the valve seals with proper lever height. There are some potential problems that can occur:
1. If the spring is too weak, or the seat or orifice is damaged, the orifice must be screwed in too far which sets the poppet back too far which lowers the lever excessively reducing the working range of the valve and reducing the flow rate in addition to increasing inhalation effort.
2. If the spring is to strong, the valve will seal when the lever is still too high meaning it will be depressed when the diaphragm and cover are installed, requiring the orifice to be turned in even farther to set the poppet back farther and lower the lever. This results in a normal working range, but can increase the inhalation effort more than is acceptable.
Ideally, the tech will make the initial air on orifice adjustment before the diaphragm is installed so he or she can see how things are looking without the diaphragm in place and can then know whether the valve is not sealing at a given point due to a seat/orifice issue or an excessive lever height issue. Knowing that will tell the tech whether spring replacement or an IP adjustment may be in order (higher IP helping to accommodate a slightly too strong spring while a lower IP would assist a slightly weak spring.)
A tech who has been indoctrinated to lower the IP on the Mk 25 in all cases to 120-125 psi for improved cold water reliability (a very negligible improvement and in my opinion very bad, or at least incomplete, advice by Scubapro) could potentially create this problem as in effect with a reg with a spring that is already on the edge of being excessive, the spring pressure would now be excessive requiring a deeper set orifice, lowering lever height and reducing working range in addition to increasing inhalation effort.
The non adjustable unbalanced SP regs are a "dual adjustment" design where orifice and lever height are independent of each other. In most cases (no dirt corrosion, salt, etc, present) leaving the spring,seat stem and inner part of the inlet fitting together and ulta sound cleaning it as a unit once the seat and orifice end of the valve has been removed is preferable as in most cases the new seat will be the same thickness as the old seat and will deliver maximum performance with no need to adjust the lever or adjust (and consequently replace) the lock nut. When possible, I think it makes sense to have parts that have been working happily stay together rather than deal with the issues and wear that arise when they have to reorient themselves to each other. However in cases where the inhlation effort is badly off from what it should be or where excessive lever adjustment is required, complete assembly and diagnosis of potential problems is in order.
