Tuning FX Impact Mk2/M3 - slower first shot - no reg creep - quasi-scientific analysis

If the reg pressure is the same…my guess would be a sticky valve. 

There have been many posts on this some have stated if I remember correctly to turn up hammer spring pressure some the peek poppet should help 

Hi guys,

Welcome everybody. This is my first post here.

I have a problem with my Impact Mk2 PP .30. The reg pressure is 145 bars. When I shoot shot after shot then my v0 is 260m/s. However, when the gun sits for a couple of hours then the first shot is 250m/s despite the exact same reg pressure - I use a Sekhmet digital gauge.

Things I did:

- hammer and hammer weight polishing,

- valve rod polishing,

- valve poppet made of peek



I do not have any other ideas.

Can you help me?

Thanks!

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The valve return spring shows this phenomenon sometimes. 

Just install a very light valve spring with the rubber ball inside and retune the gun and share the results. 

Umair Bhaur 

Ok, I have a Huma valve return spring. I will try it.

Could you explain why it can make a difference?

P.S. I truly believe every Mk2 owner has the same issue.

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Looks like that some hard springs show the tendency of becoming a bit softer when continuously in use.

But if you keep them idle for a few hours they become a bit harder again. So a few initial shots suffer. 

Metallurgy related matter. 

A very soft spring doesn't focus this phenomenon to much.

Bhaur

Interesting. The same can be applied to the hammer spring then.

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Yes but we have to find the combination of springs that works for us.

Interesting. The same can be applied to the hammer spring then.

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Futher tune your gun just around short of 10 to 15 fps from max fps by just adjusting fine hammer and valve.

Bhaur 

When I got my mk2 impact it was tuned to 140 bar reg. 
I also noticed the first shot being off consistently.

I wrote it off as reg creep, and rebuilt my reg. 
however, later on I retuned the gun to shoot on the knee of the reg pressure. That appeared to help the most. 


mk2 impact valve is design to perform best, between 110-120 bar…

If it doesn't do it for a few hours wouldn't one be best just dry fire the first shot the install the mag and load that would make sure your good on the next shot I understand that could be a pain to remember and it's a bandaid probably better just to retune or add just a little more hammer to get it in to the acceptable fps spread range

Might it be stiction of the valve stem O-ring?

The only gun I have with an O-ring on the valve stem is a B50, a Daystate Huntsman clone. It suffers from a slow first shot after sitting idle for a while. Very much the same symptom typically associated with a creeping regulator except this one has no regulator. I suspect it is due to stiction of the O-ring to the stem. On the one hand it’s surprising that the breakaway energy lost to it would be enough to matter given the massive hammer and spring on a B50, but on the other hand it’s the only conventional PCP I’ve ever owned that has this slow first shot symptom. One of these days I intend to tear it down and experiment to pin down the exact cause so right now it’s just a theory. The blame may lie somewhere else.

Meanwhile all my other PCPs have the usual slip fit between the stem and valve body. It would take a pretty sloppy fit to cause any meaningful loss of air during the few milliseconds the valve is knocked open. I ran the numbers on a hypothetical 2.5mm valve stem and a 0.002” oversized valve body. The resulting gap is only 1.6% of the area of a 4mm exhaust port and that’s a pretty poor fit. Most real-world examples would be better than that so generally speaking there is no need for an O-ring.

Yes, it can be like that but I need to find the reason just to learn.

@corny, which valve return spring did you use back then?

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All oem.

Impact's been around for many years. It's weird the issue hasn't been really analyzed.



There are actually two orings on the valve stem. One is right next to the magazine (NBR70) and the second one (PUR90) is where the stem exits power plenum. The second one does not have any groove. It sits there freely and it seals against the conical area. I suspect that bad boy.

https://www.exida.com/Blog/Stiction-Fact-or-Fiction-Start-Time-Prediction

We need to remember that PUR90 is under the pressure of 140 bars. So it's the starting friction between PUR90 and titanium as the rod is made of it.

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Probably has to do with the overall design. So many groups on utube show this issue. First shot being in a different place as the actual group. 


firing one into the dirt isn’t really a good fix…as when u hunt you might be waiting an hour or more to finally have a chance at placing a shot…only to miss.

firing one into the dirt isn’t really a good fix…as when u hunt you might be waiting an hour or more to finally have a chance at placing a shot…only to miss.

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Amen. A minor annoyance for target shooting. A total deal breaker for hunting and pest control unless the distances can be kept modest. The guns I use for that purpose I check and re-check and triple-check and then check again before squeezing the trigger on something with a pulse.

My Prod does it, my Maverick does it and my Wildcat does it.

I thought it was just SOP. I fire a shot of air at the car when I go shooting or hunting.