Air Arms Please I need help with Air Arms S400 power

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PCB

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Jan 3, 2025
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Hi everyone. I have an air arms s400 .177 that's maybe 15 years old but only shot maybe 10 tins of ammo through it. It was unused for many years and now that I've taken it out to shoot again, the power is very low. It's shooting 570fps with .5g pellets which is 5ftlb back in the day it was shooting 11ftlb

Things I've tried with little change...
Various pressures from 100 to 180bar
Spacer on firing spring to increase force
Polished main guide and bushings
Replaced orings and checked for air leaks
Checked pot depth is 57mm
Checked poppet valve and even made a new one from peek
Different pellets
Verified barrel port alignment
Verified valve guide rod is clean and free

I'm reaching out cause I'm at a total loss as to what it could be. It's consistent within around 5fps. Anything else I should check?
 
PCB said:
Hi everyone. I have an air arms s400 .177 that's maybe 15 years old but only shot maybe 10 tins of ammo through it. It was unused for many years and now that I've taken it out to shoot again, the power is very low. It's shooting 570fps with .5g pellets which is 5ftlb back in the day it was shooting 11ftlb

Things I've tried with little change...
Various pressures from 100 to 180bar
Spacer on firing spring to increase force
Polished main guide and bushings
Replaced orings and checked for air leaks
Checked pot depth is 57mm
Checked poppet valve and even made a new one from peek
Different pellets
Verified barrel port alignment
Verified valve guide rod is clean and free

I'm reaching out cause I'm at a total loss as to what it could be. It's consistent within around 5fps. Anything else I should check?
Click to expand...
It’s an S400, so the firing valve body shouldn’t have a silver power knob, but it should have a power restrictor, a pointed set screw.

Take a look at the port hole:

81F5F67A-EEB0-4C85-B1E8-E532D58FC1D1.jpeg
 
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^^^Beat me to it. We posted at the same moment.


Is the s400 the same as the s400mpr? I have the mpr version that comes set at 6ftlb. There is a set screw on the side with hex screw under that and is basically a transfer port adjuster. Possible it has turned? On the mpr removing the inner gives full 12ftlb.

Would love to see pics of the rifle.
 
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wildcj5 said:
^^^Beat me to it. We posted at the same moment.


Is the s400 the same as the s400mpr? I have the mpr version that comes set at 6ftlb. There is a set screw on the side with hex screw under that and is basically a transfer port adjuster. Possible it has turned? On the mpr removing the inner gives full 12ftlb.

Would love to see pics of the rifle.
Click to expand...
Yes, that’s probably it, W sir.
 
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Yay found the problem. The oring between the bolt carrier and valve body was squashing into and restricting the port. This happened even on countless disassembles and reassemblies. I took the barrel out to look down the grub screws hole to see what was going on. Eventually made my own washer out of a rubber glove and that's working perfectly. I'm at 12ftlb now. Happy camper
 
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PCB said:
Yay found the problem. The oring between the bolt carrier and valve body was squashing into and restricting the port. This happened even on countless disassembles and reassemblies. I took the barrel out to look down the grub screws hole to see what was going on. Eventually made my own washer out of a rubber glove and that's working perfectly. I'm at 12ftlb now. Happy camper
Click to expand...
Glad you found the issue and thanks for the follow up resolution
 
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