General Category > PCP, C02, and Helium Powered Airguns

Need info on why my QB78 suddenly developed a bad Co2 leak.

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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)

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.

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.

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!

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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