Tuning Impact Gradual Power Drop

[Asa-J]

Hi All,

Experiencing a curious issue my Impact MK2...

I don't use it that often but it has had around 1500-2000 shots through it. It has a slug power kit with DT port installed.

Just got it out and checked it and the speed was low - I wanted to reduce the reg pressure anyway so depressurised it all, then re-set reg to 140. Found the power wheel on 3 was giving 970fps with 25gr slug. I adjusted the valve spring in from 4 to 3.5 lines and was getting very good consistency at around 950fps for a bit... then watched the speed gradually drop every few shots - telling myself it's settling, but then after 70 shots over about 40 minutes or so it got as low as 900, and I hadn't changed anything else. I did notice the reg had crept up a bit over 140 but barely noticeable.

I was attached to an air line so not fallen off the reg.
Temperature had not changed during this time.

Sorry if this has been covered elsewhere - I couldn't find anything.

Would be very grateful for any advice on this. Many thanks in advance.

Asa

 

[Ca_Varminter]

I sounds like you at the edge hammer pre-load and valve spring tension balance point
So the slight rise in reg pressure, as the Reg was settling, will cause the drop in velocity.
So it a matter of either dropping the reg cutoff pressure to where it was shooting where you want it.
Or bump up the hammer preload a tad...to overcome the higher pressure..
Good Luck

 

[Asa-J]

Thanks mate, very good information!
I will make some adjustments and see what I can achieve...
Very much appreciated

 

[bigHUN]

Check the C1 got loose on that small thread

 

[Vetmx]

Yep, definitely a C1 symptom. Especially if you have the slug kit spring in there. You might be in for a surprise when you take it apart.

 

[Asa-J]

Extremely grateful for your replies.

Got it apart yesterday and the C1 was definitely not locked any more... wasn't loose loose but I could just turn it with my fingers so no doubt was moving with the shots. I couldn't remember exactly where I set the protuding section out of the body when I installed the slug kit, but I suspect 6-6.5mm, and found it now to be over 7.5mm.

Wanted to check if this was the only issue (and check in here) before loctiting it.

Set it to 6mm protuding out, dropped reg to 110.

Immediate result was power back up and sounding reasonably good, pretty sure will tune down fine to 950fps.

So will double check that measurement, try that bit of oring trick and loctite, then leave to cure.