Author Topic: Once reliable .25 Sumatra loss power  (Read 261 times)

steveoh

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Once reliable .25 Sumatra loss power
« on: January 21, 2018, 12:36:40 PM »
I’m trying to get used to this ATN x-sight but my Sumatra is all over the place.

So I set up the Chrony and with these JSBs 25.39, I’m starting out with 3k psi and at full power making under 600fps . Interestingly as I increase number of shots, FPS is increasing. I’ve put through a hundred or so pellets and scratching my head. This rifle usually makes way more power.

Sumatra has been sitting for a couple months. I did store at full pressure and am wondering it I’ve messed up the valve stem or something.

Low 554.7
High 745.0
Avg 647.7
Es 190.3
Sd 64.61

12. 745
11. 737.2
10. 706.8
9.660.5
8. 681.5
7. 660.4
6. 640.3
5. 644.9
4. 588.7
3. 596.7
2. 555.5
1. 554.7

Setting to low power had me at under 300fps.  I’ve rotated the power wheel multiple times. At one point I got a shot or two at 900fps, but that’s still low.

Ideas?


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Alan

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Re: Once reliable .25 Sumatra loss power
« Reply #1 on: January 21, 2018, 01:38:51 PM »
Wished I could help.

I did listen to Bob Sterne—theories, not actually—and ended up reducing the regulator down to 1,800 PSI, and increased the preload every so slightly. The end result, the velocity increased ever so slightly, but the shots per fill nearly doubled. I hope you find the same "knee".
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Alan

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caniborrowsomeammo

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Re: Once reliable .25 Sumatra loss power
« Reply #2 on: January 21, 2018, 03:03:19 PM »
I would check your hammer hanging/dragging.
Korean guns are a PITA to take apart put together IMHO.

Now that I'm thinking about it I should put mine up for sale, I haven't shot it in a couple years ;D
Buncha BB guns that I don't get to shoot as much as I like. Building duck boats now that I'm "retired".

steveoh

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Re: Once reliable .25 Sumatra loss power
« Reply #3 on: January 21, 2018, 03:54:48 PM »
I would check your hammer hanging/dragging.
Korean guns are a PITA to take apart put together IMHO.

Now that I'm thinking about it I should put mine up for sale, I haven't shot it in a couple years ;D

Yah. I guess I gotta take it apart. Bah.

Nooooo, don’t get rid of Sumatra!
  • Benicia, California
Quackenbush .58 Outlaw
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Sam Yang 909s .45
Sam Yang Sumatra .25
RWS Diana 350Magnum Compact Pro .22
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Diana Model 27 (childhood airgun)
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rocker1

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Re: Once reliable .25 Sumatra loss power
« Reply #4 on: January 25, 2018, 04:13:28 AM »
More than likely the poppit stem has driven through the poppit.      buy you a poppit  buddy.  David

steveoh

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Re: Once reliable .25 Sumatra loss power
« Reply #5 on: January 25, 2018, 09:30:55 AM »
More than likely the poppit stem has driven through the poppit.      buy you a poppit  buddy.  David

I have two on the way, as well as fresh o-rings all around, as well as a fresh hammer spring to cut a little off and see if I can't tame the thirst for air, and get a flatter string. Fingers crossed I got all the correct sizes of o-rings.

I have the stock off, and am looking at all the bits and pieces. I don't like the looks of the pot metal, or complexity of all the pieces and parts.  But I'll take my time.

  • Benicia, California
Quackenbush .58 Outlaw
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Sam Yang 909s .45
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RWS Diana 350Magnum Compact Pro .22
QB-79 .177
Crosman 1322
Crosman 1377 - HoRodded 10 FPE
Diana Model 27 (childhood airgun)
Tolman Skiff