Just. my 2 cents - Does your Crown have an added hammer weight? My stock MkII in .177 does not have a separate hammer weight. The hammer, which is common with the Mavericks and Wildcats, has a mass of 16.4 g. With the standard 56mm FX spring you might not have enough energy (spring compression x combined hammer mass) to open the valve enough when you raise the reg pressure above 120 bar. The .25 Maverick comes with an additional hammer weight that just drops in with a mass of 10.5 g. You can get lots of different hammer weights with different masses (up to 25.9 g) and different head sizes (which changes spring compression).
Please note that you will have to buy the Crown Slug Kit from FX or Huma to add a separate hammer weight if you don't have one. That's because you have to use a different model of spring carrier when using the additional hammer weight. Look carefully at the schematic for the Crown available from the FX website and then look at the kit on the Utah Airguns site or Huma site. You will see the difference.
I don't know what the level of your Crown Tuning skills are, but it might be worth a look at "Sub12Airgunners" YouTube channel and Search for "FX Crown". The father/son team that make the videos did a whole series on the Crown (UK Sub 12 ft-lb version, but the same principles apply). Their ideas for finding the "knee" in the velocity curve are very useful.
The Crown is different from the Impact, Maverick, and Wildcat when your Crown has the 23 stop Power Wheel. In this case each stop on the power wheel is 0.2 mm of spring compression. Each revolution of the Internal Hammer adjuster is 0.5mm of spring compression. So with the Crown you set the IHS adjuster to some length (I used 4 mm), and then pick a starting setting on the Power Wheel where you have room to go up or down at least 4 clicks. That way you can fine tune with the power wheel to get the muzzle velocity you want. If you can't get the velocity and your max PW setting shows velocity is still increasing, then open up the rifle and turn the IHS two revolutions in. Back off five clicks on the PW (puts you at the same amount of spring compression) and try again.
Tuning is actually a kind of logic puzzle that is very satisfying when you figure it out. Have fun!
Cheers,
Greg
P.S. - I guess that was more like a buck fifty than two cents - oops! Sometimes I can't help myself from going into professor mode.
