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Slow fps Impact X 700mm .25

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Avidrob

Member
Mar 25, 2019
34
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Netherlands
 

Impact .25 700mm

Jsb king mk2 34gr

Factory settings yeild low fps. At 145bar i can get to 830-850.

My Current settings are 

165bar, VS 8.5mm, HST 17.5mm

This is giving me 870fps with an SD of 2.5 and an ES of 8, very happy with accuaracy at the speed but it seems very high reg pressure for that velocity. 

What can i do, i see some folk here are geting that speed at 135-140b with a 600mm barrel.

Is it possible that the 700mm barrel needs a bigger plenum, and with out is less efficient than the 600mm. Or is it a tight barrel?

All seals are good, no leaks/blows c3 bumper good, as is hammer and valve seat.

Id like to get to 870 at 140 bar, is this possible?




 
Do you have the hammer spring adjuster set for maximum compression/ pre-tension?

You want it like the bottom pic.

1558824555_12465482705ce9c66b9912a5.24086988_IMG_20190116_131214.jpg


Also, check this video for the best procedure.

https://youtu.be/9K9uG2KOPt0


 
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Guages are already wika, and they have been verified with a digital guage.

Hammer load tension isnt maxed out, but beyond the plataeu at 17.5mm yields no more fps, and causes more bounce un needed vibration, and inconsitant speed and accuracy. For more speed i need more reg pressure together with more Hammer tension

Iv been tunning a long time so i know the process, im happy with the results just not happy with the numbers. 165b for 870fps 34gr seems too high
 
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Looks like you need more hammer weight , at the expense of lock time and vibration .
Click to expand...

For any given reg pressure Iv been tuning a the hammer tension to the fps plataue, further increasing tension or hammer weight doesnt give any more fps, it just over strikes the valve, and means the power wheel does nothing. 



At 165b with 34 gr this plataue is 870 fps 

At 145b this plataue is 830 fps.

I dont need more fps i just want 870 fps at 135-140b.

What reg pressure vs speed are other 700mm .25 users getting?
 
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Look at valve rod ( in magazine loading area) it should be flush. Could indicate valve seat problem. Its an Fx Impact. Like owning a Lotus! Constantly adjusting and adapting.
Click to expand...

Its spot on and flush, recently replaced valve rod and valve seat.

The only thing i havnt checked is how well the probe, and valve body transfer ports line up. Is there s way to see this with your eyes or is it just a case of taking measurements.

Pellet probe measurment from rear block is within spec.
 
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You say you have the VS at 8.5mm? To me that looks like it maybe limiting the valve movement? Have you made sure you have turned it all the way out? On my gun, a impact X, it measure much more than that when turned out beyond the 4th ring.
Click to expand...

HST was tuned with the VS all the way out giving about 885 fps, VS was then turned down to 8.5mm to drop velocity about 15fps, to tighten up consistancy.
 
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You say you have the VS at 8.5mm? To me that looks like it maybe limiting the valve movement? Have you made sure you have turned it all the way out? On my gun, a impact X, it measure much more than that when turned out beyond the 4th ring.
Click to expand...

HST was tuned with the VS all the way out giving about 885 fps, VS was then turned down to 8.5mm to drop velocity about 15fps, to tighten up consistancy.
Click to expand...

Ok, you may have to back it all out again when trying lower pressure. When using lower pressure, the valve may has to move more to let more air get by. If you keep it is same position, power may just drop when lowering pressure.
 
Upvote 0
You say you have the VS at 8.5mm? To me that looks like it maybe limiting the valve movement? Have you made sure you have turned it all the way out? On my gun, a impact X, it measure much more than that when turned out beyond the 4th ring.
Click to expand...

HST was tuned with the VS all the way out giving about 885 fps, VS was then turned down to 8.5mm to drop velocity about 15fps, to tighten up consistancy.
Click to expand...

Ok, you may have to back it all out again when trying lower pressure. When using lower pressure, the valve may has to move more to let more air get by. If you keep it is same position, power may just drop when lowering pressure.
Click to expand...

Totaly, but when i tuned at 140b, i hsd the VS all tje way out and velocity would max out at 850, then the VS was adjusted to get to 830. 

What speed and pressure do you get with 34gr king mk2s?
 
Upvote 0
You say you have the VS at 8.5mm? To me that looks like it maybe limiting the valve movement? Have you made sure you have turned it all the way out? On my gun, a impact X, it measure much more than that when turned out beyond the 4th ring.
Click to expand...

HST was tuned with the VS all the way out giving about 885 fps, VS was then turned down to 8.5mm to drop velocity about 15fps, to tighten up consistancy.
Click to expand...

Ok, you may have to back it all out again when trying lower pressure. When using lower pressure, the valve may has to move more to let more air get by. If you keep it is same position, power may just drop when lowering pressure.
Click to expand...

Totaly, but when i tuned at 140b, i hsd the VS all tje way out and velocity would max out at 850, then the VS was adjusted to get to 830. 

What speed and pressure do you get with 34gr king mk2s?
Click to expand...

My .25 barrel is rather new, so I for the moment just shoot the 25 grain on moderate speed as I still has some lead fouling issues. Have never tried the 34 grains yet, so I do not know.
 
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When I first chronyed my impact X with the 700mm .25 barrel which had a regpressure of 115 bar set for shooting my previous .177 barrel, the velocity peaked somewhere above 950 fps, with the .25 grains. The only thing I did was to increase hammer spring tension and back out valve stop. From there on I have both lowered speed, and regpressure, as it was more than I needed. I know it is not comparable to a 34 grain though. I btw have a edgun digital gauge measuring my represure, so I believe it should be pretty accurate.
 
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UPDATE.



So i ported the Probe and brass inlet yesterday,

This gave some improvment but still not amazing,

Best i could get was 890 fps at 165b Hst17.8 Vs wide open.

With the VS tuned this came down to 870. 

At 145b and vs wide open max fps was 858. 

Am i expecting too much ? I realy dont want my reg this high as shot count at 165b sux

Wondering if there are slight variations in valve housing/ breach bock TP tollerances that could explain the varistions we see from one rifle to another. 

Is it worth milling out the TPs on the valve/breach block?

Any opinions Ernest?
 
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