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

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Seeker

Seeker

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Jan 23, 2022
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Europe, Poland
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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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!









Click to expand...

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 
 
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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.

Click to expand...

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
 
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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
 
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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.
 
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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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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.
Click to expand...

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.


 
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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.
Click to expand...

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.
 
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