Author Topic: Regulated gun tuned on the down-slope  (Read 199 times)

mcoulter

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Regulated gun tuned on the down-slope
« on: February 12, 2020, 05:31:11 PM »
Hello Bob,

Am I correct in my thoughts that having a gun showing a shot string similar to your down-slope example in the link below, that my two options are to either increase HS tension or decrease regulator pressure?  (With the goal being to try to eliminate the slow rise in velocity and then the surge at the end of the string.)

https://hardairmagazine.com/ham-columns/tuning-regulated-pcp-airguns/

Background - I picked up this used Cricket .25 that came with a 4.5 cm plenum extension and a rather fancy hammer spring adjuster that also included an extra weight that sat behind the hammer.  This adjuster was causing some extreme binding and I was able to replace it with one that was more traditional and did not include the weight.  I suspect that the actual hammer spring is probably standard, but I don't know about the valve spring.  The regulator is a huma and came to me adjusted to 120 bar. 

Here is a recent string:

https://photos.app.goo.gl/7QWLs7CCSj5WjRkVA



I also suspect that I'm still not getting a consistent hammer hit.  I have noticed that the plating on my replacement adjuster is starting to peel off and may be causing some of these smaller fluctuations from shot-to-shot.

Oh, here are some pics of the original hammer and the extra weight.  The weight looks like a nail and the "head" fits inside the back side of the hammer.  It is actually really well made but with my current spring, it binds badly.  Is I'm writing this, I'm wondering if it may be worth trying a shorter spring with that adjuster.







Any suggestions would be most appreciated  :-)

« Last Edit: February 12, 2020, 05:42:07 PM by mcoulter »


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rsterne

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Re: Regulated gun tuned on the down-slope
« Reply #1 on: February 12, 2020, 09:17:38 PM »
You are correct in your assessment.... If you want more power, increase the hammer strike.... If you want the same or less, reduce the setpoint pressure and then retune....

The large shot-to-shot variation while above the setpoint is typical of a downslope tune.... as the valve is operating in partial valve lock.... If you tune closer to the knee, that should decrease....

Bob
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mcoulter

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Re: Regulated gun tuned on the down-slope
« Reply #2 on: February 13, 2020, 01:05:15 PM »
Great, thank you Bob  :-)
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mcoulter

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Re: Regulated gun tuned on the down-slope
« Reply #3 on: February 23, 2020, 05:16:47 PM »
I dropped the regulator from 120 to 113 bar, tested various hammer spring tensions, and then shot a couple of full strings. 

First, here is the HS test at various tensions in my .25 Cricket (with plenum extension):



I ran my first string at seven turns on the hammer spring which looked good on the graph above, but in reality accuracy was disappointing at 890 to 900 fps with the .25 33.4 grain pellets.  I had much better accuracy making a .5 turn change and shooting again at 6.5 turn on the hammer spring.  Here is the string:

https://photos.app.goo.gl/irtpT1RPTSt3wMmR8



The surge in velocity was mostly gone or perhaps somewhat less pronounced at the end of string, but there are still a few shots with low velocity.  On shot #37 I really could feel what seemed to be a light hit on the hammer.  I could tell even before looking at the chrony that it did not sound right.  Would valve lock present itself that way?  As I write this I am looking at the 7hs velocities and I guess I don't see the same drops...  Hmmm....

So I guess that I'm talking myself into dropping the reg a *little* more to get the hammer spring tension up to about 7 turns. 

Perhaps dropping it from 113 bar down to 110.
« Last Edit: February 24, 2020, 09:43:30 AM by mcoulter »
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Mrshosted

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Re: Regulated gun tuned on the down-slope
« Reply #4 on: March 06, 2020, 09:19:37 PM »
Did you ever have a chance to lower the regulator pressure?  I'm interested in hearing how your tuning going
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rsterne

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Re: Regulated gun tuned on the down-slope
« Reply #5 on: March 07, 2020, 09:38:09 AM »
Sorry, I missed your post on Feb. 23.... Yes, you are starting to see velocity swings, and still have a slight rise at the end of the string, because you are tuned a bit too low on the knee.... Normally I would just go back to HS = 7, but if your accuracy is not as good at that higher velocity, then dropping the regulator pressure a fraction is your only solution....

Bob
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mcoulter

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Re: Regulated gun tuned on the down-slope
« Reply #6 on: March 08, 2020, 05:44:57 PM »
NOTE >> Since this is not really an "Ask Bob" thread any more, I'll make any future posts in the PCP board.

Hi folks,

The answer is yes I did drop the reg, but it’s a bit complicated  ;-)

My Cricket had developed a slow, but annoying leak, so I took it apart this weekend to reseal it.  I changed the reg pressure while it was apart, replaced the little white delrin valve disk inside it, and reg’s orings too.  This was all fine, no surprises here.  HOWEVER, I also pulled out the gun’s valve stem and I didn’t pay  close attention to how it was preloaded.  I wrongly assumed that the screwed in disk that compresses the valve spring was simply screwed in to bottom out.  Well it wasn’t.  So now I have one more variable to have fun with.  On top of it, I also suspect that replacing the reg’s new seal disk also changed regulator set point. 

Observations:

Good news.  The gun’s velocity seem more consistent.  I didn’t have the big drops I had seen before.
It’s less efficient (maybe 3 mags from a full fill vs. 4 and it sounds VERY much like it’s wasting air) and on top of it, the gun is shooting a bit slower.
I saw a slow drop towards the end of the string and not any surge in velocity like before.  In fact, I shot all the way down to 87 or so bar (reg is set at a labeled 110 bar).  (hmm)

Sorry, I don’t have the transcribed shot string yet, but here’s a pic of my numbers.  The light, since it was late in the day was poor, so there are some missing shots.  The first mag of 12 shots sounded very noisy at 5 turns on the HS.  So the next mag I dropped it to 4.5 turns on the HS (less tension). The gun sounded a bit better and velocity didn’t drop much at all. 

String:


Valve preload - I think I need more preload on the spring


Valve spring - after watching Ernest's videos, I suspect this is a replacement spring that's weaker:


Reg stem (it's hard to see the orientation of the washers - I have left them as-is and you can see the white seal disk on the right side):



Reg length (just noting this before I took things apart):


Target - this is the good news.  The gun DOES shoot really well at these velocities.  These are 12-shot groups at 55 yards.  As you can see the last group on the bottom left kind of fell apart.











« Last Edit: March 09, 2020, 09:42:20 AM by mcoulter »
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