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 Post subject: Qb78 valve tune
PostPosted: Sun Mar 03, 2013 9:27 pm 
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Joined: Sun Mar 03, 2013 7:35 pm
Posts: 7
Location: porthope, ontario
Hello there, I am new to this forum and am just getting back into air guns. I have searched for the past hour trying to find out information as to how to tune a qb78 valve and haven't come up with anything. Could someone point me in the right direction, it would be most appreciated. I have a 3000psi bottle regulated to 1200psi that I will be using to get started. I have all the equipment to do the work, I lack the knowledge of what to do to get started! My thanks to anyone who get get me started in the right direction.
Jeff


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 Post subject: Re: Qb78 valve tune
PostPosted: Sun Mar 03, 2013 9:32 pm 
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Joined: Mon Sep 05, 2005 10:55 pm
Posts: 6845
Location: Rocky Mtn Hse Alberta
reduce valve spring tension an 1/8" or so
remove the filter material and screen.
Space up the hammer spring say 1/4" , make a sleeve that fits over the guide and will easliy enter the hammer.
should get you there.

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Walter


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 Post subject: Re: Qb78 valve tune
PostPosted: Sun Mar 03, 2013 9:43 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8973
Location: Coalmont BC
I've found that the biggest restriction is getting more air into the valve.... At a minimum you need to remove the piercing pin and filters, the sleeve and washer.... That leaves the spring much too short, so you need to find a longer one, to reach all the way to the front of the valve.... Drill a shallow 5/16" recess there to locate the end of the spring....

There are a couple of ways to open up the front of the valve.... You can drill some holes, or mill or grind some slots in the sides of the front section, like this....

Image

Note the change to the transfer port.... The stock port is often off center and squashed.... Both the valve and the breech have a small 1/4" diam. flat milled in them to accept a poly tubing transfer port.... The barrel port is opened up to match, and make it line up with the hole in the breech.... as they are often off center.... You can also carefully clean up the sharp change in direction in the valve exhaust port with a Dremel....

At 1200 psi, you shouldn't need to change the preload on the hammer spring, and in fact doing so will likely just waste air with little or no increase in velocity.... Shimming the hammer spring is only required to open the valve at higher pressures.... Replacing the flo-through bolt nose with an extended probe (3/32" diam. on a .22 cal is fine) will also increase the flow and add some velocity.... at the expense of less room to load the pellet....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Qb78 valve tune
PostPosted: Sun Mar 03, 2013 9:51 pm 
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Joined: Fri Nov 10, 2006 12:02 am
Posts: 1075
What Walter said and also take out peircing pin, the steel tube and change brass valve stem for Delrin one. You can open up t/p in barrel and bolt also.


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 Post subject: Re: Qb78 valve tune
PostPosted: Mon Mar 04, 2013 12:35 am 
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Joined: Sun Mar 03, 2013 7:35 pm
Posts: 7
Location: porthope, ontario
Is there any other type of valve work done if one will be using higher
pressures? I likely won't ever exceed 1500 psi but would like to know
How far one can go with the valve work. I am going to complete the valve work and
use the gun for awhile before I start looking at hammer springs etc. I don't mind if
the gun is a bit of a gas hog, better that then having to pull the gun all the way down
again for more valve work. Thanks, jeff


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 Post subject: Re: Qb78 valve tune
PostPosted: Mon Mar 04, 2013 11:07 am 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8973
Location: Coalmont BC
If you want to go more extreme on opening up the front of the valve, you can cut more away, like this....

Image

Not only will the flow be less restricted, but you will gain a small amount of volume in the air reservoir as well.... Whether the additional available airflow will be of any benefit will depend on how well the air can get out of the valve and into the barrel.... In addition to the things I mentioned previously (valve exhaust port, transfer port, barrel port), you can also bore out the back of the valve slightly around the head of the poppet, and turn down and taper the head of the poppet....

You don't seem to have a very good understanding of how the hammer spring preload works.... If you use 1500 psi (your original post said 1200) you will have to increase the preload on the hammer spring to make any gain from the additional pressure.... Otherwise, increasing the pressure past about 1200 psi will decrease the velocity rather than increase it.... On the other hand, going too far on the preload will simply waste air.... At 1500 psi, between 1/16" and 3/16" (2-5 mm) should be about right with most QB springs.... Add washers (about 1 mm thick) until you don't gain any more velocity and then take one out.... Here is how preload affects velocity....

Image

Note that as you add preload past a certain level the velocity no longer increases but the amount of air used does, and hence the efficiency (and number of shots) drops....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Qb78 valve tune
PostPosted: Mon Mar 04, 2013 9:10 pm 
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Joined: Sun Mar 03, 2013 7:35 pm
Posts: 7
Location: porthope, ontario
Thanks again for the info. I can see how important the spring preload is. Is there a solid design out
there utilizing a spring adjuster. The bolt hole goes straight through the middle of the endcap. I have looked and
really found one adjuster that 1. Looked like it would do the job 2. Looked OK on the gun.
Thanks,
Jeff


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 Post subject: Re: Qb78 valve tune
PostPosted: Mon Mar 04, 2013 10:35 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8973
Location: Coalmont BC
If you modify the gun to "cock on open" then you can use a simple adjuster like this....

Image

However, particularly if you use an extended bolt probe and shims on the hammer spring, you will have difficulty loading pellets.... That is the reason for the extra notch at the back of the bolt slot to hold the bolt open....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Qb78 valve tune
PostPosted: Tue Mar 05, 2013 12:17 pm 
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Joined: Tue Jun 03, 2008 5:46 pm
Posts: 2814
Location: Canada
Modified striker solves the issue of loading room with cock on open. :)

Bottom one is in the QB79.

Image

Al


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 Post subject: Re: Qb78 valve tune
PostPosted: Fri Mar 08, 2013 8:08 pm 
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Joined: Sun Mar 03, 2013 7:35 pm
Posts: 7
Location: porthope, ontario
I really like the power adjuster idea. I have it drilled and ready to go. I am trying to come
up with a knurled knob and lock idea. It will have to adjusted by Allen wrench or screwdriver,
Just thinking of a way to make flow with the rest of the gun. I haven't done the cock to open mod yet.
Maybe tommorow. I have the gun torn down and most of the above changes made. I started a new post
but I likely should have kept this going, new to forums and get lost on them pretty quick. I have ordered the
3000psi/1400psi output regulators from Jessie so I am committed at that pressure for now. What material do
you guys use for the valve plug seal. I want to still be able to run co2 in a pinch but am wondering if nylon or
Another material would be best used. Is there anybody in Canada making. Valve stem/plugs? Thanks for the help!
Jeff


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 Post subject: Re: Qb78 valve tune
PostPosted: Fri Mar 08, 2013 9:08 pm 
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Joined: Sat Nov 03, 2007 4:19 pm
Posts: 8973
Location: Coalmont BC
Al, I know the top striker is stock in a QB78.... are you saying the bottom one is stock in a QB79?.... I've never seen one like that....

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Qb78 valve tune
PostPosted: Tue May 21, 2013 12:41 am 
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Joined: Sun Jul 15, 2012 10:49 am
Posts: 6029
Location: Victoria, BC and Clarkston, WA
Bob? Could we link this with the One we started Sunday morning? I know you are not a moderator, but making this a sticky for the QB7X would help all the newbies like myself!

This one:
http://www.airgunforum.ca/forums/topic43563.html
and this:
http://www.airgunforum.ca/forums/topic55307.html?hilit=QB79

Oh Lauchlin?!?!?!
PLEASE!

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Pardini K10
Brocock Concept .22
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 Post subject: Re: Qb78 valve tune
PostPosted: Tue May 21, 2013 7:15 pm 
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Joined: Sat Apr 16, 2011 5:58 am
Posts: 495
Location: Eastern Ontario
Quote:
Is there anybody in Canada making valve stems/plugs?


Eric @ ScopesAndAmmo has them: http://scopesandammo.com/storefront/pro ... fles-p-886

Cheers,
Todd

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Cheers,
Todd


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 Post subject: Re: Qb78 valve tune
PostPosted: Wed May 22, 2013 12:10 am 
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Joined: Tue Jun 03, 2008 5:46 pm
Posts: 2814
Location: Canada
rsterne wrote:
Al, I know the top striker is stock in a QB78.... are you saying the bottom one is stock in a QB79?.... I've never seen one like that....

Bob


Sorry Bob, not sure how I missed this. :oops:

The 79 uses the same striker as the 78. The one at the bottom in the pic is one I made for my 79. The "nose" eliminates the extra travel from the re-positioned sear catch.

Al


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 Post subject: Re: Qb78 valve tune
PostPosted: Fri May 24, 2013 1:39 am 
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Joined: Wed Oct 17, 2007 3:27 am
Posts: 2585
Location: Winnipeg, Canada
sholo wrote:
Quote:
Is there anybody in Canada making valve stems/plugs?


Eric @ ScopesAndAmmo has them: http://scopesandammo.com/storefront/pro ... fles-p-886

Cheers,
Todd


These are much better than the soft rubber seals on the factory stems. They are lighter in weight, and react differently to hammer strike, and pressure.

I got some very flat strings with the delrin stem on CO-2 in my 78-D.
They seem to work well at 3000 PSI of air as well- in a tube designed for it

Regards,

Doc Sharptail

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