Benjamin valve mod questions

[vaquero57]

I have a late 90s Benjamin 392. Several years ago I studied up and performed a valve mod. I cut the intake valve down shorter, installed a 2nd spring, brass washer, and reinstalled the mainspring (upon recently tearing it down I cannot tell if I also shortened the main valve spring after doing this mod. It looks untouched). Then I installed a slightly oversize O-ring so the 2 halves wouldn't quite thread all the way back together. Whoever I had talked to at the time told me this would increase valve volume, decrease headspace, and supposedly make the gun easier to pump. I was also told this would make it a 10 pump gun. I didn't then and still don't own a chrono so I had no idea to see if I had gained anything. All I know is the gun had some wallop (but I suppose these guns do right out of the box so......who knows). With 10 pumps I never retained any air. I had been shooting it for a while like this when I lost the bug and put it away. None of my airguns and firearms have been shot in several years. Recently I have found a new fondness for this gun again and I decided to disassemble it to inspect my long ago mod. I discovered that my second spring (hardware store spring) was rusted. I didn't have a spare so I removed it and the washer and freshened the oversize O-ring and reassembled with just a factory valve spring and shortened intake valve. It still shoots, holds air, and performs but I now have to pre cock the hammer before pumping. I assume this is due to the shortened intake and now the mainspring is not under as much tension to resist the hammer spring.

So I am sitting here asking myself if this was all worth it? I have no idea of knowing what or how much I gained. I cannot decided whether to purchase a new spring and reassemble as a dual spring valve again or just leave as is. Can anyone chime in with real world DIY experience that doesn't involve a lathe or an engineering degree lol? I have studied and read and studied some more lately. The opinions are all over the place. Lets just say I would love to have 700+ FPS and increased energy on 8 pumps if possible or even an ACP which required fewer pumps to top back off but not sure that's attainable without fancy machine work?

[Robert 5mm]

In my opinion do not do any more mods - you performed the mini-T.Mac mod and actually having just the one spring ensures the hammer when fired will empty the valve. ( having to cock gun before pumping confirms exhaust valve is weaker then hammer spring ).

You will not get 700 fps with 8 pumps as is - the mini-T.Mac mod increased valve volume and decreased pumping effort.
You could decrease valve volume ( put a small SS pin inside spring inside valve ) - this will INCREASE pumping effort but also increase internal valve pressure and MAYBE get you to 700 at 8 pumps. ( and maybe not - that is why I first said do not mess with it )

[vaquero57]

In my opinion do not do any more mods - you performed the mini-T.Mac mod and actually having just the one spring ensures the hammer when fired will empty the valve. ( having to cock gun before pumping confirms exhaust valve is weaker then hammer spring ).

You will not get 700 fps with 8 pumps as is - the mini-T.Mac mod increased valve volume and decreased pumping effort.
You could decrease valve volume ( put a small SS pin inside spring inside valve ) - this will INCREASE pumping effort but also increase internal valve pressure and MAYBE get you to 700 at 8 pumps. ( and maybe not - that is why I first said do not mess with it )

So what would you predict 10 pumps would get me with this valve setup? I shot it with 10 pumps after the mod everytime I shot but that was with the second spring and washer installed. I should actually have more volume now without the washer and extra spring

[Back_Roads]

 Here is an inexpensive way to get rid of the guess work, and always know your gun is at its best.

[Robert 5mm]

Similar valve setup with single spring

[Van]

  I use stainless steel springs where possible in my pump gun valves.  The hardware store ones are usually rust prone, unless labeled stainless, when moisture is pumped in the valve. 

[vaquero57]

Similar valve setup with single spring

Thanks for that info. You say similar to my mod which is intake shortened, and #113 o ring that doesn't let valve quite screw all the way together? I see the word "steroid" on your charts. Does this mean your gun was steroided by Tim at Mac-1? I hear Tim does other things to his valves beside what other DIY guys like myself do with our simple mods.  Could my stock pump linkage handle 12 pumps? 10 doesn't feel that tough and I have been 10 pumping this gun every time I have shot it with no issues but never tried to go beyond 10..

[vaquero57]

  I use stainless steel springs where possible in my pump gun valves.  The hardware store ones are usually rust prone, unless labeled stainless, when moisture is pumped in the valve.

Good advice here lol!

[vaquero57]

Here is an inexpensive way to get rid of the guess work, and always know your gun is at its best.

Thank you!