GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: GumpIsrael on August 21, 2016, 03:04:30 AM
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Today the valve on my QB78 went out, classic hissing coming out of the breech when open and cocked and out of the muzzle when closed.
Just out of curiosity, is this a common issue with QB78's? Is there something I can do to beef up the valve seal? Maybe even an alternate material?
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Blown valve stem seal...need a new valve stem. Yes, you can search here for alternate materials.
More common when running of 12gr. as that's an "open system" (meaning you have to open the air tube every time you recharge with 12gr., and 12gr. carry along whatever dirt/grit/dust they stuck to them). A bit of grit blown though the valve at +900psi (co2 pressure can be a lot higher than that in hot weather) can slice the soft valve seal (after all, a typical sandblaster runs on about 100PSI). Even being extra careful when using 12gr., I still expect I'll slice a seal now and again.
Being curious, I've sliced open used 12gr. Most are pretty clean inside, but some evidently weren't all that clean inside when they were filled. Also have the suspicion that not all of the metal pierced by the piercing pin just folds back and stays in one piece.
Closed systems (bulk fill/hpa conversions) are less likely to ruin a seal, but it can still happen. Having looked inside of bulk fill co2 tanks, it's pretty evident that not all co2 is really "clean" (although the food grade stuff from beverage supply co. is better filtered...won't do to have crud floating around in you fountain served 7-up). Best to keep the connectors that fill those systems clean and plugged when not in use.
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The most common issue is contamination on the valve seat, especially in CO2 systems with the fiber filter. I have not had one fail outright. However, the poppet in a FD-PCP is virtually identical and I did have one of those fail...the seal became dislodged from the brass body.
If you need a replacement, Mountain Air is your best bet. At pressures around that of CO2, get the Teflon. At pressures over 1300psi, I would say the Delrin. In either case, be sure to burnish the popper to the seat otherwise it is likely to leak.
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Beat me to it.
Below is a picture of disassembled QB valve. Old rifle, don't know if they still put all that "&^^&" inside their valves now or not.
The little mesh filter can some times shed pieces, but that fiber filter they used back then (and may still use) is eventually going to just come apart with time and use.
Can't see a fiber filter in the picture?
That's because to totally gave up and is wrapped around the piercing stem.
(http://i157.photobucket.com/albums/t50/ribbonstone/valves/DSCF0001-10_zps828303c9.jpg) (http://s157.photobucket.com/user/ribbonstone/media/valves/DSCF0001-10_zps828303c9.jpg.html)
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The filters are still put in the brand new guns as far as I know, (I think mine was made sometime last year). But I took mine out not long after I got the gun. I shot it quite a bit when it was in the low 90's this summer. I'm betting the presure is what killed that seal. Cuz every now and then it gor left out in the heat.
For now I have an old valve stem I can throw in there, but only while I wait on my new one to arrive from mountain air. I may make this an hpa eventually, so i'll probably go with the delrin.
Thanks ya'll!
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I shot it quite a bit when it was in the low 90's this summer. I'm betting the presure is what killed that seal. Cuz every now and then it gor left out in the heat.
Elevated pressure won't damage it in my experience. Several HPA conversions have been operating with the factory poppet at 1300 - 1700psi for 2-3 years and many, many thousands of shots.
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Interesting Jason, I wonder what the failure was caused by then, maybe just a flaw when it was made?
My experience with QB's is only one year's worth so I admit I have no great knowledge about them.
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It's certainly possible it was factory defect. What did it look like when you pulled it? Did the seal come out of the poppet?
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Are you guys sure it's the valve stem seal and not the seal between the valve and the tube? The latter is made from material that absorbs a lot of CO2 and it will make that seal bubble like soda water thereby damaging it. Mine didn't last past 1 CO2 charge. Had to replace it. Now my QB will hold pressure for months and hasn't leaked since. Ordered them at GCP (https://woodfield-gcp.co.uk/?product=qb78-qb79-seals-bolts-kits) in the UK.
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The seal looked rather dented and had what looked like circular rings dented into it, but it wasn't loose or anything.
Yes, Josan, most definitely the valve. Yesterday I took it out and replaced it with another that I had laying around and it's working fine now.