Tuning Impact MKII velocity is too high

[Corvid_hunter]

I bought a MKII .25cal 700mm with a Power Plenum and ended up changing the port and probe to the Huma versions. I can't use a moderator and my 150 bar tune with the hammer spring .5mm from being maxed out gives me velocities in the mid 800 range on MIN with .25gr Kings (valve at line 2, 1050 or so on MAX). If I turn the valve in (2/3 turn from closed) I get around 763fps and nice accuracy on MIN and only gain about 20fps on MAX. I'm thinking this can't be good for the gun (for some reason or another that I don't know about). I'm thinking of taking the gun down to 90 bar or so and starting over so I can get around 700fps on MIN and 900fps on MAX with the Power Wheel operating as it should. If I go this route, is there any reason why my rifle won't be quiet and efficient? Without de gassing the rifle, how much can I reduce the reg pressure per drop (with shots in between to let it settle)?

edit....checked some older posts and dug out my manual and found the info with regard to lowering the reg pressure.

 

[golfer70]

I have the same gun. It was shipped new to me with the reg set at 105 bar and it shot the 34 grain JSB pellets at 905fps. I hope this helps determine a starting point. Enjoy. 

 

[biohazardman]

Remember to remove the tank and then purge the air from the reg and plenum before you adjust the reg down. 

I memory serves a higher reg pressure and a shorter duration on the valve being opened will get you the efficiency and quietness you are after more than a lower pressure with the valve opened longer. 

 

[Jayson]

Remember to remove the tank and then purge the air from the reg and plenum before you adjust the reg down. 

I memory serves a higher reg pressure and a shorter duration on the valve being opened will get you the efficiency and quietness you are after more than a lower pressure with the valve opened longer.

Take note on this!, well said! A lot of hammer and low pressure = common and! I don’t know how so many people got there?

 

[tor47]

You could also try the original pellet probe, and see if it slows it down some, as it has a smaller hole.

 

[Glem.Chally]

Turn it down then.

No need to degas the gun to turn it down as it's not an MK1. Quarter turn and burn off a shot or two is all you need to do. Rinse and repeat.

Also all that preload is not necessary. .5mm from max preload is way too much.

Try unscrewing that set screw until it just slightly moves that slider between MIN and 1. Go from there would be my advice.