40 bar sweet move change???

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Terry1
Posts: 14
Joined: Mon May 11, 2020 3:15 pm

40 bar sweet move change???

Mon Jun 08, 2020 8:30 pm

Evening everybody.

Recently plucked up the courage to fully strip , clean , replace o-rings , etc on my s410 tdr (200mm cylinder).

Did a full chrono first with .22 jsb rs and the sweet spot was at 150 bar for 19 shots at around 604 FPS.

After polishing , replacing springs etc the FPS is now around 620 (a success so far) but the sweet spot is 190 bar for around 20 shots. Shot it down to about 100 bar and it just follows a decreasing arc of speed / power.

I’m not complaining but just a little confused. Is it normal for that much of a change?

Oh and before I forget I must say a huge thank you to Julian for the time and confidence that he gave me to turn a gun that I really like into a table of bits then back again

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tonyc
Posts: 376
Joined: Mon May 29, 2017 10:55 am

Re: 40 bar sweet move change???

Mon Jun 08, 2020 9:49 pm

Are you seeing the same number of shots overall, and the indicated pressure dropping gradually - in a sensible way for number of shots... so no obviously false pressure display?
Have you changed the firing valve or valve spring?
Was this the first fill after the service? Have you run a subsequent string over the chrono?
I'm just wondering if something is settling in after the service.

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Terry1
Posts: 14
Joined: Mon May 11, 2020 3:15 pm

Re: 40 bar sweet move change???

Tue Jun 09, 2020 6:31 am

About the same number of useable shots (20ish).

New hammer spring, firing valve , firing valve spring and pot. Pot put in at 57mm.

The pressure drops in line with the shot count.

Did a second chrono after about 150/200 shots fired through and it showed the same, sweet spot at 190bar and a spread of around 7fps for the first 20 shots. Power / speed then decrease to become unusable at around 150 bar

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tonyc
Posts: 376
Joined: Mon May 29, 2017 10:55 am

Re: 40 bar sweet move change???

Tue Jun 09, 2020 7:26 pm

Are you sure you are getting a true reading from the gauge?

Something not making sense: if you are filling to 190 and running down to 150bar how can you be getting the same overall number of shots? If you are only getting 2/3rds of the original number I'd say your gauge is defective and you are only filling to around 150bar.

The more usual shift in the peak, caused by deformed valve, is towards lower pressure. Also any freeing up of the hammer over a cycle would result in the power creeping up. The only way I can account for a shift in the other direction is (a) increasing friction of hammer/rail, (b) increasing resistance of the valve, or (c) air starvation at lower cylinder pressure. Have you done anything to reduce the air hole into the pot?

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Terry1
Posts: 14
Joined: Mon May 11, 2020 3:15 pm

Re: 40 bar sweet move change???

Tue Jun 09, 2020 7:43 pm

I think the gauge is reading correctly. It’s a brand new hills mk4 pump whereas I was using an old stirrup pump but the pressure reading on stirrup pump gauge and cylinder gauge read the same.

The only other thing that is different post and pre strip down is that I fitted a lane depinger. It’s a tight fit with two o rings to hold it to the cylinder wall and it has two offset 1mm holes in the plastic “baffle” section

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tonyc
Posts: 376
Joined: Mon May 29, 2017 10:55 am

Re: 40 bar sweet move change???

Tue Jun 09, 2020 8:55 pm

I've no experience of depingers.
One other possibility for air starvation: if the transfer port is restricted too much it will reduce air flow disproportionately at low pressure. If the hammer spring is too strong (e.g.fitted a FAC spring?) and you throttled the TP that might be it... or backed off the valve spring pre-load and throttled the TP. Best way to set the power is with the TP unrestricted, adjust the pot and, if necessary, fine tune on the TP screw.

Edit:
I just had a look online at the design of the Lane depinger. You have put it in the right way round and checked it clears the pot? If it contacts the end of the pot it would almost certainly restrict the air flow into the pot, which would then possibly be the cause.

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Terry1
Posts: 14
Joined: Mon May 11, 2020 3:15 pm

Re: 40 bar sweet move change???

Wed Jun 10, 2020 6:08 am

Very good point about the de pinger. I’m fairly sure it clears the pot but I’ll take the cylinder off when I get a minute and double check. It’s got a rod which is used to space it off of the firing valve body but I must admit that I didn’t check before installing

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