Removed the brass firing pot.

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RichieRich612
Posts: 128
Joined: Fri Apr 19, 2019 2:02 pm

Removed the brass firing pot.

Tue Jul 09, 2019 10:31 pm

I removed the tube from the pot which extends into the air cylinder and found my Mpr is more consistent.
There is a tiny bit more of a twang when firing a pellet but it's made a big difference to velocity consistency.
Has anyone here tried this?
Richard

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TenMetrePeter
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Joined: Wed Jun 29, 2016 5:40 pm
Location: Luton Beds UK

Re: Removed the brass firing pot.

Wed Jul 10, 2019 9:59 am

What did you do with the valve spring S306?
If there wasn't one and you have no reg fitted then that may have been the problem.
The tube adjusts in and out to tension the valve spring. Quite interesting it is consistent at all.

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RichieRich612
Posts: 128
Joined: Fri Apr 19, 2019 2:02 pm

Re: Removed the brass firing pot.

Wed Jul 10, 2019 11:55 am

TenMetrePeter wrote:
Wed Jul 10, 2019 9:59 am
What did you do with the valve spring S306?
If there wasn't one and you have no reg fitted then that may have been the problem.
The tube adjusts in and out to tension the valve spring. Quite interesting it is consistent at all.
The screw in section which houses the valve stem and spring is still there.
Only removed the elongated section which protrudes into the air cylinder.
Richard

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TenMetrePeter
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Location: Luton Beds UK

Re: Removed the brass firing pot.

Wed Jul 10, 2019 12:57 pm

OK thought it was one piece in those.

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RichieRich612
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Joined: Fri Apr 19, 2019 2:02 pm

Re: Removed the brass firing pot.

Wed Jul 10, 2019 1:11 pm

TenMetrePeter wrote:
Wed Jul 10, 2019 12:57 pm
OK thought it was one piece in those.
So did I!
No they are 2 pieces held together with loctite.
There's a faint line roughly 5mm above the threads, just slips off with some heat.
Lot better now.
Richard

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TenMetrePeter
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Re: Removed the brass firing pot.

Wed Jul 10, 2019 2:29 pm

My 6ftlb MPR doesn't have the extension . I’ve never got to dismantle it and they’re so rare there’s no images online either.

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tonyc
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Re: Removed the brass firing pot.

Sat Jul 13, 2019 11:16 pm

I suggest you chrono a full string from 190 bar to 90 bar with and without the pot then you will see how it functions. It's possible if you chrono just a few shots (say 10) you will see more consistency but that will be coincidental depending on the cylinder pressure. Removing the pot shifts the peak of the power curve to a lower cylinder pressure but makes the curve more pronounced by causing the valve to close more quickly at higher cylinder pressures. For example, if you chrono a S400 with and without pot at 120-110 bar, with the pot the velocity is coming out of the sweet spot and falling, without the pot it's still in the sweet spot and relatively flat.

The other effect is the peak power will increase, so anyone removing the pot (or the pot falling off as mine did) from a gun that was running at 11.5ft lbs will probably find it running at 12+. I've no experience with the MPR, just the S400, but the principle is the same. I chronoed several full strings trying to figure out what was going wrong. I had a series of power variations on the S400 which I finally found was caused by the two sections of the pot coming loose and the volume changing before the end section eventually became completely separated.

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RichieRich612
Posts: 128
Joined: Fri Apr 19, 2019 2:02 pm

Re: Removed the brass firing pot.

Sun Jul 14, 2019 6:04 am

tonyc wrote:
Sat Jul 13, 2019 11:16 pm
I suggest you chrono a full string from 190 bar to 90 bar with and without the pot then you will see how it functions. It's possible if you chrono just a few shots (say 10) you will see more consistency but that will be coincidental depending on the cylinder pressure. Removing the pot shifts the peak of the power curve to a lower cylinder pressure but makes the curve more pronounced by causing the valve to close more quickly at higher cylinder pressures. For example, if you chrono a S400 with and without pot at 120-110 bar, with the pot the velocity is coming out of the sweet spot and falling, without the pot it's still in the sweet spot and relatively flat.

The other effect is the peak power will increase, so anyone removing the pot (or the pot falling off as mine did) from a gun that was running at 11.5ft lbs will probably find it running at 12+. I've no experience with the MPR, just the S400, but the principle is the same. I chronoed several full strings trying to figure out what was going wrong. I had a series of power variations on the S400 which I finally found was caused by the two sections of the pot coming loose and the volume changing before the end section eventually became completely separated.
Yes that's exactly what I did.
Full chrono string of 100 shots and found my sweet spot.
Power did slightly rise but easy to tune back to legal limits.
It's my h.f.t rifle now after tuning myself to be more consistent than air arms shipped them out!

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rob2hook
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Re: Removed the brass firing pot.

Mon Sep 09, 2019 5:19 pm

It is my understanding that the part you have removed acts as a sort of plenum chamber giving some restriction to the flow to the firing valve at the higher pressure end of the scale. This also fits in with the multi variable balance of parts and adjustments in this remarkably successful non-regulated design. It seems from what you say the you have been lucky in that removing it has effected an improvement. I wouldn't expect this to be the case in all or indeed many cases. IIRC Nobby, on the AGF, has set up his S400 for HFT by doing the opposite and has reduced the hole size to increase its effect on the firing cycle. He does say though that it would not be appropriate in a hunting scenario as pressures might not equalize quickly enough for a follow-up shot.

Rob.

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RichieRich612
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Joined: Fri Apr 19, 2019 2:02 pm

Re: Removed the brass firing pot.

Mon Sep 09, 2019 9:09 pm

rob2hook wrote:
Mon Sep 09, 2019 5:19 pm
It is my understanding that the part you have removed acts as a sort of plenum chamber giving some restriction to the flow to the firing valve at the higher pressure end of the scale. This also fits in with the multi variable balance of parts and adjustments in this remarkably successful non-regulated design. It seems from what you say the you have been lucky in that removing it has effected an improvement. I wouldn't expect this to be the case in all or indeed many cases. IIRC Nobby, on the AGF, has set up his S400 for HFT by doing the opposite and has reduced the hole size to increase its effect on the firing cycle. He does say though that it would not be appropriate in a hunting scenario as pressures might not equalize quickly enough for a follow-up shot.

Rob.
Thanks for your lengthy reply Rob.
It's got me thinking about what Nobby did by reducing the diameter on the plenum hole to "Improve the shot cycle".
I think reducing or increasing the hole makes no difference at all.
The pressure within the cylinder at full charge is immense and would easily fill the plenum volume instantaneously regardless of hole diameter. We're talking tons of pressure here at 190 bar.
Besides, the airflow would still pass around the threads of the plenum as they're not sealed at all just there to set the plenum in position.
Another factor to take into account is that the plenum is just a way Air Arms have simply designed a way to adjust compression on the valve spring.
Therefore it's purpose is only to adjust the valve opening.
It's a crude design and that's why the transfer port adjuster is the best way of improving the shot cycle.
My Mpr has no plenum so it's operating at it's best now, straight from the cylinder.
No plenum also increases cylinder volume, so much in fact that even with a carbine cylinder fitted it gives me 100 useable shots as opposed to the advertised 70 by Air Arms for a carbine cylinder.
Rich

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