3000 psi tube QB78

[remtom1200]

So?....do I use the pretty white seal for the valve...or the black one with the PCP conversion? Do I lube the barrel and valve o-rings for assembly or no? I have been reading and have no firm answers...lol.

[AirGunEric]

White seal is for the stock Co2 cap- it will be a tougher fit getting it around the valve and in the tube- so use the black one for the valve. Don't mind the POS seal the factory used- it's probably half disintegrated by removing the valve from the tube. You can lube the o-rings on the barrel- do NOT lube the o-rings on the PCP tube end or the valve with anything but a silicone lube- petroleum by-products in a PCP are bad news.

[remtom1200]

Thank you Eric, and I made the valve POS seal a pos taking it out....he he. PCP for me tomorrow!

[remtom1200]

Well, ...interesting! At 1500 psi it sounds like a 22LR but there is something not right. Following the shot it sounds like i have a load of air traveling through the barrel after the pellet has left. I am guessing the valve is staying open too long? I had modded the valve and made a pedestal for the spring and might not have put enough tension on the spring, as well put a spacer about 3/16 on the hammer side. I am now left with the question do I remove the spacer or crank up the valve spring tension?. I have no numbers to share at this point but it shoots very strong. Shot count is way down as well, around 12 shots from 1500 - 650 psi

[maecolis]

It sounds like your hammer is hitting too hard and keeping the valve open... I would reduce the force of the hammer spring and/or up the valve spring force.

Normally, the hammer spring has just enough force to propel the hammer and exceed the force required to open the valve (which is resisting with valve spring plus compressed air). If the valve spring isn't strong enough, it doesn't have enough strength to close the valve once the hammer hits it. If the hammer spring doesn't have enough strength, or the valve spring is too strong, it doesn't open the valve.

I wouldn't be surprised if you are going supersonic with your pellets... and you're losing a LOT of air.

[AirGunEric]

I'm not sure why you modified the valve for use at 1500psi? Not necessary and has gotten you were you are now. Can you return it to it's stock form?

[SeanMP]

Agreed.

You are flowing huge amounts of air that's not doing any work. My best advice is return all the springs back to stock and see what you have.

Usually the valve needs some custom work on the inlet side to make use of any spring mods. Yes it will flow air but the openings are restrictive which causes a huge pressure drop in the valve. So the large flow is not effective.

Here is a treatise by the Master of Volume and flow

http://www.airgunhome.com/agforum/viewtopic.php?t=7674

[remtom1200]

I guess I should have gave some more details on what I did and why 1500psi. My Valve is milled top and bottom with 5/16'' slots, valve stem tapered and a delrin seal insert. I thought ahead of the new pedestal mount for the spring and made it adjustable if needed without removing the valve by threading the original puncture pin hole 1/4''-28 and threading the pedestal . Barrel port was opened to a #18 hole and bolt probe was installed. 1500 psi was a test for leaks and I couldnt handle the suspense had to go outside and try it at -10c...lol. I was hoping to run this setup in the 2000ish psi range once I get the springs working in sync. You could be right about the supersonic. Heavier pellets than the cphp are on the list.

[SeanMP]

Awesome. That sheds more light on things.

What I have noticed is that QB's respond rapidly to increasing spring preload. A little bit goes along way. 3/16" combined with lowering the valve spring pressure was probably too much.

I would bring the valve spring up to original or maybe even more. Then add in hammer preload. At lower fill pressures the valve needs some help to snap closed.

If I turn in my adjuster 1/16 (0.067) from where I have it set for 480fps the result would be high 600's

[remtom1200]

I just may reach up in there with my very long screwdriver and give the spring a little more pressure. Is it possible to separate the valve while in the tube and change out the spring(if necessary) without removing the whole valve or damaging the o-ring? Or is this not advisable?