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FX Crown mkII velocity/fps issues


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Hi All,


I have a FX Crown MKII .177 -it's about 3 years old. It's only modification is I put in the Huma expanded transfer port as a friend had a spare one and also put in Huma guages. (edit for clarity: I installed this mod a long time ago and with it, achieved my very consistent 11.8ft/b tune which is documented in the imagery below. It has nothing to do with my current issues) 


I'm now getting inconsistent velocities. Please see image below with the 50 shot string. 


The chrono readings with the 2.2 SD were after i'd tuned previously (months ago) using the 95% method, but documented at this point in time to show the 11.8ft/b tune.


The 50 shot string with the 6SD & 25fps variation was after leaving the gun for a few hours, to document the disparity after leaving the gun. 


You'll notice at this point it is going over 12ft/b for a few shots (one shot is 12.6!) - due to ... well, whatever the issue is!


I have made the gun inoperable by removing the whole hammer spring system.


It goes without saying but I am aiming for a tune below 12ft/b and in no way wish to modify my rifle to be over 12 ft/b in any way, shape or form.


Thought: I did switch to a new tin of pellets around this point, the same brand and type (JSB Heavy 10.34 grain 5.52) Could this be that I have a bad batch of pellets, with some being lighter than others?


I have a feeling it could also perhaps be an issue with the regulator. Is there any easy way to remove the reg and clean it?

Thankyou all. 



unnamed (1).jpg

unnamed (2).jpg

Edited by Baddaboom
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Extra: I have bought some QYS 9.56 grain pellets to see if they produce a better and more consistant velocity/fps - if it still has this issue with those then i'll know it's not the pellets but an issue with the reg.

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Posted (edited)


I don't think I quite explained it properly. Adding the Huma transfer port was a mod I installed a long time ago, it's a fantastic mod and has nothing to do with power but air conservation. 

I previously had a tune which produced 11.8ft/b, for a long time. This is with the Huma transfer port.

The recent mad ES/SD, which appears to have been taking me over the limit on a limited number shots, is a recent event/occurrence, unrelated to the transfer port. 

That's why I believe it may be a regulator issue, or perhaps due to a bad batch of pellets with inconsistencies in weight. 

Also, you'll note as I have removed the hammer spring system, that my rifle is not an illegal air rifle as it is not assembled or functional. I am seeking measures via this post to make sure that once I assemble it again and set the reg/HST up, it will be mechanically stable to stay under the 12ft/b limit. 

I hope this clears it up and answers your questions. 

If anyone would like to comment further with helpful advice rather than unhelpful lauding, It would be very much appreciated.

Edited by Baddaboom
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So looking at your shot strings above .on the 2nd of March  you were getting around  714 fps with little a 9 fps spread then 12 hours later ,next day you were getting around  730 fps with and a spread of 23 fps .

So this sounds to me like a hammer spring issue .I'm no reg expert . But the reg (measures out a set pressure into the plenum ) and the knock open valve takes air from that plenum instead of the main reservoir. When a reg tends to fail the washers start to stick and the pressure in the plenum either fills too slowly (fast shooting can result in low power shots )  or it doesn't fill the plenum to the correct pressure  = the odd low power shot. If the reg is sticking open then you will be  pulling air  directly from the main reservoir + plenum and this can give higher power  readings .  But if the hammer spring isn't consistently  pushing the hammer and knocking open the main firing valve then the best reg in the world isn't going to give you consistent readings. If the hammer has too much oil/lube it can alter the speed that it is released .the spring can bind and then release shot to shot (look at the ends  it may be broken  )  the fact its been left over night and possibly warmed up suggests that lubrication somewhere may be the issue  .

Look to the hammer assembly first  its easier to diagnose and fix  . Check the knock open valve is OK also .orings and return springs .then lastly the reg .

Regs tend not to like being adjusted too much .the more you mess with it the less reliable it will become (usually  ) 

If the reg is at fault then its either a reg service or replacement  .

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Hiya Ultrastu,

Thanks so much for your comment.

My hammer spring mechanism including the spring looks fine. I run it without lubrication. 

How would I check the valve?

Do you know if a reg replacement on the crown is do-able?

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I have refitted the Hammer spring system and turned the HST down by three clicks on the side dial on the crown, and it's now doing well, a SD of 2 again and a steady 11.7 ft/b.

Not a clue as to why it decided to jump up without me touching it so that now i have had to turn it down three clicks for it to go back again.

I'm going to leave the gun in a cold room and then come back to it when it's cold and see if the velicty has changed. 


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I think that  if your hammer spring is so easy for you to adjust , then maybe its possible that it's self adjusting .though they usually tend to wind out so power goes down not up .but I don't know your mechanism exactly  .

If you have just altered it and the sd has dropped to 2   I feel that this is the place you should be looking and not the reg . See how it is tomorrow and report back .

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Next question .does your gun have any sort of short curve though it's entire shot Count .similar to an un regged gun ? Ie starts lower on power  raises to a maximum  energy around 150 bar then drops off slowly till its below acceptable  power levels .?  If so then the reg is stuck open and your shooting of main cylinder pressure ..

Also just looking at fx crown mk2 images  they have a 3 stage transfer port adjuster. Does yours have this and does it work and what's it set at and does it make a difference if adjusted .

Shot to shot sd will usually be better if a narrow t.p. is selected over a large one . I think .

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The hammer spring system is a fully adjustable unit, the wheel turns on the outside of the gun up and down. Obviously you need your max number on the wheel to be 12ft/b so it can't 'easily' be turned up to go over. 

No the gun doesnt have a shot curve. the reg is working, as the crown has an outside regulator gauge that resets after each shot like it's supposed to. 

I have to select the large TP on the TP dial. If i selected small and tuned to 12ft/b, then plod could come alot, turn the wheel two points and it would be going massively over - same issue as with the HST wheel. ALL need to be on max and putting out 12ft/b

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OK just out of interest  if you select a different tp setting is the gun consistent  or does the energy creep up then ? .

Maybe your knock open valve is sticking open longer than it should giving higher readings .this would probably be due to either .A. weakening valve spring .B a sticky valve stem (corrosion possibly ) or  C . a deformed valve head .

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I changed my amp reg in my old impact mk11 to a jeffsky unit off the air gun forum. It worked perfectly and was loads more consistent. They have a YouTube channel sub12 air gunners 

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