Tuning FX Impact

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Frank4889

Member
Sep 20, 2019
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La Romana, Dominican Republic
Hi everyone,

I have a .25 MK Impact Power Plenum, i can tune it to where it is extremely constant but if i set the rifle down for 5 minutes it will drop 10 fps from my standard tune, if i let it sit for an extra 10-15 minutes after that shot it will go down another 10-15 FPS.
after 4-5 shots its will be back to normal. I put in a new regulator, new valve housing and valve rod. The issue persists. I dont know where to look anymore. It had never happened before. I think this issue is no good for hunting or pest control since i would have to be shooting constantly to not get this drop in fps. Any input would be greatly apreciated. 
 
Impacts usually have some reg creep and that could be what’s going on with you gun. As long as the reg creep isn’t too bad you can usually fix the problem by increasing hammer spring a little bit. I learned this “trick” from an old post by Ernest on this forum. After re-tuning my gun I no longer experience velocity drop on my first shots, even after letting gun sit for months without being used.
 
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Example Tuning Process

  1. For a given reg pressure, you need to increase hammer spring until your velocity plateaus. You're looking for the top of bell curve or maybe a little past it. This is needed to make sure your first 1-2 shots are within the ES, after your gun has been sitting for a while.
  2. Shoot 3-shot groups over the chrony at each power wheel setting starting at MIN, to determine which power wheel setting the velocity stops going up.
  3. Once hammer spring is set, use valve adjuster to fine tune velocity.
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    Example - let’s say your RP is set at 125 bar. Start with the power wheel on MIN. Let’s say the velocity peaks at setting 3. Settings 4, 5, and MAX do not increase velocity. Turn your PW back to 3. Use valve adjuster to reduce velocity down within reason (20-40 fps). If your speed is too hot, then decrease RP and start over. If speed is to slow then increase RP and start over.

    When your gun sits for a while without being shot you may notice a little reg creep. This can be considered "normal", but should be less than 5 Bar. If reg creep is more than 5 Bar then you may need to service your regulator to get creep back in check.

    Note: As RP is increased, more hammer spring pre-load may be needed to open the valve. With the gun un-cocked, turn the power wheel to MAX and adjust the hammer spring tension screw CW in 1/4 in turn increments. Shoot a few shots over the chrony each time you make an adjustment and keep doing this until velocity no longer goes up.
 
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Man really appreciate the input. Ive been going crazy over this issue. My reg keeps steady at 90 Bar which is where i usually leave it. I guess i will hace to re tune the gun. What i was doing was locking up the hammer tension and increasing reg pressure till i got the fps i wanted, once i had that fps i would back out the hammer spring until i experienced a small drop in fps. 
guess i was going about it backwards
 
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Start with your valve adjuster all the way out (i.e. line 4)

There is a lot misinformation out there where guys are telling you to max your hammer spring and hammer spring pre-load as “Step 1”. I found that to be a misleading approach. 

I’m shooting JSB 18.13 gr pellets at 880 fps with reg pressure at 81 bar, power wheel on ‘E’, and valve adjuster at roughly 2.5 lines. My hammer spring pre-load is set so the cam barely engages when i go from MIN to 1 or E. I should mention that I do not have the power plenum on my Impact.
 
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Man your info was spot on, i tuned like you said and its not budging. Waited for a half hour and tried again and it was literally spot on first and second shot. I am shooting the 26.8 Nielsens at these velocities and getting awesome results. You cant imagine how much i appreciate the help. Bellow is my last test, shots 8 and 9 are after a half hour wait



A0B1AD75-8C9C-4F75-9D52-4C51BE316E7E.1616194775.jpeg

 
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Start with your valve adjuster all the way out (i.e. line 4)

There is a lot misinformation out there where guys are telling you to max your hammer spring and hammer spring pre-load as “Step 1”. I found that to be a misleading approach. 

I’m shooting JSB 18.13 gr pellets at 880 fps with reg pressure at 81 bar, power wheel on ‘E’, and valve adjuster at roughly 2.5 lines. My hammer spring pre-load is set so the cam barely engages when i go from MIN to 1 or E. I should mention that I do not have the power plenum on my Impact.
Click to expand...


I was watching a Crown MKII tuning video on the AEAC Vlog where Steve called it a neutral cam setting when the cam barely moves between MIN and 1. That is his recommended way to tune that rifle.
 
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