Author Topic: Need info on why my QB78 suddenly developed a bad Co2 leak.  (Read 846 times)

22_boomer

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 :-[ I went to do some target practice and could only get about 4 shots out of the rifle (I only use a single charged cartridge) and it got a bit worse down to just 2 or 3 shots. I'm deaf among other things so can't hear a leak. I went to Archer Airguns and bought the tune up kit for Chinese QB78 rifles and hoped replacing the seals that the kit includes will fix the problem. Am I working in the right direction? What else should I look for when I have the rifle apart?

EDIT to add -- I am also disabled and that's the reason I only use one Charged cartridge in the rifle. Even with that I can only get off maybe 20 shots at the most and I'm done for the day. If there is pressure still left in the pressure tube I leave it and in the past it would hold for a few weeks so I was totally surprised when the rifle started loosing pressure as soon as I started to use it. If leaving the pressure in there is a bad thing please jump on me for that.

Thanks in advance,
Boomer (more like a hiss now)
« Last Edit: May 04, 2021, 02:22:09 PM by 22_boomer »


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Alan

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #1 on: May 04, 2021, 03:28:50 PM »
I haven't a clue where you're located, so you might want to clue us in to a close by city. Hopefully, someone here will have the wherewithal to help you personally.
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Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

steveoh

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #2 on: May 04, 2021, 04:59:11 PM »
I have a QB78 and they will tend to leak if you don't put a drop of Crosman oil on the CO2 cartridge when you install it. It's important to not over tighten the CO2 cap, and to just rotate firmly and then back off 1/4 turn.

They will leak other places too. You can put a balloon over the muzzle to see if there is a leak there. The balloon will partially fill, and stand proud if so. That usually means it's an o-ring at the valve.

They will sometimes leak around where the barrel inserts into the breach. Soapy water will show that.

They will also leak around the o-ring in the sealing cap for the CO2 tube.

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22_boomer

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #3 on: May 05, 2021, 05:18:21 AM »
Steveoh,
I hope that kit has all the O rings I'll need. I will have to admit that I believe I read the about backing the cap for the Co2 tube a 1/4 turn but forgot all about it. I have always tried to put some Crosman oil on the tip of the un-pierced Co2 cartridge since I believe it helps to lube parts. I had put some of the Crosman lube on the O ring on the bolt and hope that was not a mistake. I'll see if my wife can buy a small pack of balloons to fit on the muzzle to see if it shows a gas leak in that area of the mechanism. I hate the idea of using soapy water on the action but If that kit doesn't fix it then I don't have a choice. Thanks for the information!
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22_boomer

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #4 on: May 11, 2021, 05:43:32 AM »
Steveoh,
I got the XP Tune kit and it works great but the leak is still there so I tried your advice. I put some soapy water on the Co2 chamber cap and it did a nice job of blowing bubbles. I tried to locate replacement O rings for the cap since when I checked the existing one with a magnifying glass (Bad eyes) it looked like it was full of cracks. I just happened to have a box with all different sizes of O rings that I had for who knows what and the largest fastest one seemed to fit the cap so I installed it and charged the rifle. It's been two days and the rifle is still charged so maybe I fixed it. I'm not sure of the O ring being compatible with Co2 but so far it works. I checked with Archer Air Guns for the O ring but he was out of stock. Amazon has some but I guess I don't want to pay for something when I found one that works and I still have more than a dozen of them left. Thanks for that soapy water idea -- at first I didn't want to get water even close to the rifle but the soapy water located it in an instant. The rifle was already out of the stock so I wiped everything down to dry it, moved the barrel band out of the way since water was under it and then wiped it all down with CLP.
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steveoh

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #5 on: May 11, 2021, 06:53:40 AM »
Good deal!

On my QB78 that o-ring is a strange thick white material. I think I last used a o-ring from a Harbor Freight kit.

But eBay has some: https://www.ebay.com/p/1188495220

One guy on GTA says to use a urethane #113.

Then there’s this: https://www.gatewaytoairguns.org/GTA/index.php?topic=53689.0

The recommendation being Mac1 Airguns milspec o-rings.

https://www.mac1airgunshop.com/mobile/default.aspx

Mil Spec Nitrile
Duro  =  75D

AirGun O-rings are a pain as much of the challenge is finding the correct sizes and material.
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Alan

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #6 on: May 11, 2021, 07:48:53 AM »
CO2 isn't picky about o-ring seals, so I suspect you're okay.
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Alan

I have a Hill EC-3000 compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your tank as a courtesy (4,250 PSI limit).

22_boomer

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #7 on: May 13, 2021, 10:46:46 AM »
Alan,
The Co2 isn't picky but I found the O rings are. The O ring kit I have has a large O ring that fit the cap perfect and the Beeman held the gas like forever. I read that some O rings can absorb the Co2 and swell up and start to come apart. That is exactly what my O ring did -- now this is the one on the cap that seals the Co2 gas tube and is under pressure till it's all used up. After shooting the rifle till the gas ran out I went to take the cap off to check the O ring seal and put in a fresh Co2 cartridge. The cap was hard to get off and the O ring had really swelled up and was starting to crack. I looked all over for a good O ring size 113 that was not too hard, durometer 75 and I found this article that listed the Co2 cap Viton O ring from McMaster-Carr number 9464K26 so I ordered them. I hope these work.
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steveoh

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #8 on: May 13, 2021, 11:27:58 AM »
I have several paint ball co2 tanks that use Urethane O-rings. I've not seen them swell up. But those funky white 0-rings the QB78's come with will swell, and get sticky as though they've come in contact with a solvent.  Curious.
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22_boomer

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #9 on: May 13, 2021, 02:27:03 PM »
I have several paint ball co2 tanks that use Urethane O-rings. I've not seen them swell up. But those funky white 0-rings the QB78's come with will swell, and get sticky as though they've come in contact with a solvent.  Curious.
Yes, the original white O ring that came with the rifle started to look funny and finally gave up. That was the cause of the bad leak I had. The problem then was to find an O ring that wasn't affected by Co2 under pressure for a long time.
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nervoustrigger

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #10 on: May 13, 2021, 07:16:03 PM »
Steve is spot on regarding polyurethane.    It is the best choice for the end cap.  You can use Buna-N or Viton for permanent seals but they will swell so they will be a nuisance on the end cap.  Another option is to swap in a fresh O-ring when changing cartridges, and set the original aside to outgas and return to its original size. 
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22_boomer

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #11 on: May 14, 2021, 05:11:47 AM »
Steve is spot on regarding polyurethane.    It is the best choice for the end cap.  You can use Buna-N or Viton for permanent seals but they will swell so they will be a nuisance on the end cap.  Another option is to swap in a fresh O-ring when changing cartridges, and set the original aside to outgas and return to its original size.

This is the problem I ran into -- some people say use this and some say use that. I'll keep in mind the polyurethane O rings if the ones I ordered still give me trouble. I just wish that one of the parts sellers that deal in Co2 rifles would sell the rings. Most of them seem to have a variety of O rings with no real explanation as to what they can work with.
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steveoh

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #12 on: May 14, 2021, 06:02:40 AM »
https://www.alliedmetrics.com/products/o-rings/metric/types/

Quote
POLYURETHANE O-RINGS

Polyurethane withstands extended contact with CO2, is physically tough, and features good extrusion and abrasion resistances. However, it is vulnerable to heat damage and can only tolerate environments that don’t exceed 100° C. Common applications for Polyurethane O-rings include:

Valves
Cylinders
Pneumatic systems
Hydraulic fittings
Firearms
Fluid transfer systems
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22_boomer

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #13 on: May 14, 2021, 09:38:53 AM »
Steveoh,
I appreciate you finding that information. I can order the correct size O rings in polyurethane but they only have them in 70 hardness and 90 hardness and I think 90 would be a bit too much for me to install. Would you thing the 70 hardness is OK?
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nervoustrigger

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Re: Need info on why my QB78 suddenly developed a bad Co2 leak.
« Reply #14 on: May 14, 2021, 01:03:24 PM »
Yes, 70 durometer is slightly more preferable for the end cap. 
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