Tuning Crown Mk II help needed

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gfmiller58

Member
Sep 10, 2020
118
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Virginia, United States
The set-up is Crown Mk II, 25 caliber 600 mm barrel, regulator set at 130 bar, front power wheel on High and rear power wheel on 23. Until yesterday this set up would push a JSB 25.39 grain pellets at 1015 fps. Yesterday if was shooting low. I cronographed it today and it was hitting less than 900 fps. The gun has air, it isn't leaking. I've tried to diagnose the problem, to no avail.

The rear power wheel was not behaving normally to adjustments. It is one of the new 1 to 23 wheels. The cam lobe at 23 chipped. Put the old max/min wheel back on. Still slow. Tried adjusting the regulator pressure. Still slow.

Removed the stock to check the hammer spring adjustment. A small grub screw fell out of the action / stock. It is 10 mm long, 3 mm in diameter and accepts a 1.5 mm Allen key. Where does this belong?

The front power wheel also feels "off". There is almost no indent at the High setting. The gun actually gains 20 fps if the wheel is slightly past the indent. Is this front wheel serviceable? When I tried removing the Allen bolt that holds it on, it just turned the knob and did not un thread.

Right now, I don't know what to do. Hopefully you guys can steer me in the right direction. Thanks.
 
Sounds like it dropped out the hammer spring threaded adjuster. They normally have some form of thread locker on them to prevent this. Maybe got missed in manufacture? With such a high FPS, it's very likely that the 'recoil' shook that bad boy out. So, you'll need to remove the stock, maybe you haven't put it back on? Then remove the hammer spring block/slide by loosening the socket head screw that holds it in. That screw is located from the rear of the gun, just below where the cocking bolt is located/sticks out when cocking. It's a 1.5mm Allen size. Once removed, the slide will easily come out from is hole by a very gentle pry on one end from where the wheel was.

Take it out and clean both the slide internal threads and the 1.5mm adjusting screw that you found. Put a dab of thread locker on it and rethread it back into the slide. The socket end facing the muzzle the bare end goes toward the wheel location. Reassemble in reverse order and adjust the newly installed adjusting screw to the tune that you want (it likely shouldn't be all the way out though, maybe half to three quarters?). Hopefully, it'll stay in place with the fresh thread locker and you'll be good to go.

Bummer that we can't insert pictures of the schematic right now, it might make it easier to understand. Reviewing the video, at mark 4:25 you can see the slide (elongated oval, flat piece) with the screw sticking out with the bare end, the other end has the socket 1.5mm. Bob-O is measuring the gap between in that sequence.

mike 
 
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If that allen screw is the same one from the video, it should solve part if the problem. If that screw came out, it basically renders the hammer spring adjuster useless. Once it us back in, you should notice changes in velocity when you adjust the hammer spring. The other adjuster knob aligns different size ports for airflow. It sounds like the screw may have loosened and allowed one if the 3mm ball bearings to slip out of place. Have you tried pulling up slightly on the knob itself while turning the screw? Slight upward pressure might let the screw grab enough to back out. Don't pull up too hard, though. If you are able to remove it, be careful not to lose that tiny ball bearing. I bought a package of 3mm ball bearings on Amazon for just that purpose. I hope this helps.
 
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^ This…on my mk2 crown the transfer port adjuster backed off on its own and got misaligned. Took a little head scratching to figure out. Backed off enough that the detent ball fell out which didn’t help me at all. Bought a baggie of them online , 3mm I think it was, loctited the screws and reassembled with no further issue. Take FX’s advice and give them a call; they could walk you through it on the phone much better than we can here. 
 
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Follow-Up Report:

I de-gassed my Crown and removed the transfer port knob and internals. The transfer port holes inside the receiver block had gotten mis-align. When the knob was turned to "HIGH" the holes inside were slightly closed. I re-aligned the transfer port holes and put green lock tight on the retaining screw on the off side of the receiver. I also installed a new Huma hammer spring.

Without adjusting the regulator pressure the gun is now sending the JSB 25.39 gr. pellets out at 1000 fps. I'll do some testing to see if increasing the regulator pressure above 130 bars will now increase the muzzle velocity.

Thanks for all the help guys. 
 
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