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Old 08-18-2006, 08:57 PM   #1 (permalink)
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Sheridan valve advice needed

I have a P-12 that is still giving me some valve issues. It has had a CA conversion added sometime during its long life, and I believe that this is the root of its persistent low velocities. I have it running at around 240, which is barely acceptable. This is with a pretty heavy bias towards the hammer spring, and a very heavy pump stroke. I have cut the valve spring down, and I have had to cut some of the cupseal valve stem down in order to allow the valve to open completely.

When I was looking at Palmer's selection of accessories and parts, I noticed the different valve stems for CA and cartridge powered guns. I have already cut most of the 12-gram piece pin down on my stem, and I now have comparable valve movement to my PGP. However, on Palmer's site, the CA stem has had nothing past the cupseal intact. Should I take this route and completely cut off the rest of the stem? I'm worried that nothing will guide the valve spring once this is done.
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Old 08-19-2006, 02:27 AM   #2 (permalink)
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Is yours setup for a dummy 12 gram with the ASA coming off it on a 45 degree angle towards the trigger guard ?

If so, I've got one just like that ( I think someone locally did this conversion, since all the ones I've seen like that are on the west coast ) and it also suffered from really low velocity after I rebuilt the valve (it had been ruined by it's previous owner). It turns out that the valve spring in it was completely wrong. I swapped it out with the one in my Piranha SB and it jumped up quite a bit. So I went to a hardware store and bought a spring that's pretty close to the Piranha one and that seemed to fix it.
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Old 08-19-2006, 04:48 AM   #3 (permalink)
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Yes, I have that exact setup. How do the springs compare, dimensionally?
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Old 08-20-2006, 12:44 PM   #4 (permalink)
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How looks your valve body? Have you bolt with velocity adjuster?
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Old 08-20-2006, 02:05 PM   #5 (permalink)
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The valve body looks pretty much mint, and I do have a velocity adjuster bolt. However, I have the adjuster screwed all the way in so that it isn't obstructing air flow whatsoever. I assume this is neccessary to achieve the highest velocity?
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Old 08-20-2006, 05:09 PM   #6 (permalink)
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Imho it is problem with gas flow. With the CA mod you have better separated liquid CO2 from gas CO2 so you have lower velocity than by first shoots with powerlets. Imho you need simply mods for "Low-pressure" - better air flow for the "thin-bodied" power source.

You can dry boost the valve air flow even more. Entlarge the holes in stock valve body - central opening to 6.5mm, four holes to 5.5mm, round the channel bottom into R3.

Or you can Entlarge the hole between of the tubes. Look here:
http://invisionfree.com/forums/Stock...howtopic=15792

Or you can solve your velocity problem the other way. In Speed Daemon bolts there are big leaks trought thier velocity adjuster. You can remove the adjuster, seal the entire space with full screw (it is 5/16 UNF lenght 1/2"), teflon tape and tin solder (in the slot for the o-ring) and get rear velocity adjuster (adjusting the velocity trought preload of the hammer spring). After sealing of the Speed Daemon bolt, the velocity of my P68SC in his first 13 shoots increased on average by 45 fps. So when you have 240 fps now... if are 285 fps good enought for you, then I think that you can solve your velocity problem with this mod.
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Old 08-21-2006, 01:32 PM   #7 (permalink)
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I think you're right about the speed demon bolt. I have noticed a great deal of blowback through the adjuster. I may try to seal it up. Do you think simply teflon taping the adjusting hex screw would work?
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Old 08-21-2006, 06:23 PM   #8 (permalink)
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The leaks are related with the lock-balls of the adjuster: first with the lengthwise cut out in the screw for them and second with the violation of the slot for the rear o-ring in the bolt body with opening for them. So in reality, you need remove the small balls, exchange the screw for full screw, seal them in the teflon taping and seal the opening in the bolt (you can use tin solder or epoxy) to fix that totally. It is not hard.
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Old 08-22-2006, 03:17 PM   #9 (permalink)
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Sounds like a great idea. The lock balls don't really work because of the blowback, anyways.
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Old 08-22-2006, 03:46 PM   #10 (permalink)
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Start on the valve end first.... leave the bolt for now.

As 3022 stated the valve in that gun was designed to run off powerlets (liquid co2) you are now feeding it with Constant Air (gas) so you need to open up the ports in the valve.

You can either send this out to have the work done or do it yourself. This is not the easiest task, it can be a little tricky and by no means will you have a Palmer tweaked valve if you do it yourself.. they have it down to a science... on our 10 job we'd still be considered hacks.... but you can do the work.

The opening of the port between the tubes requires even more skill.

If you find after opening the ports on the valve, and using the correct springs (order the springs from Palmers for constant air guns.. they are pretty much the same though) that you still have the need for more work.. I would go with a one piece bolt from palmers.

I've never had blow back or blow by issues with my speed-demon bolts and I dread taking one apart just because they are a major pia to put back together. First try lubing the bolt with grease/vasoline (lube the inside of the bolt the expanding portion/slider not he outside) and you may see some of your "problem" disapear.

But start with the valve... that is where you will get your greatest improvement.

E
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