Author Topic: QB-78 problems  (Read 5147 times)

Offline Tpatner412

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QB-78 problems
« on: March 08, 2010, 10:14:23 PM »
Okay this is for any QB-78 owners or experts out there.  My precious 78 is leaking very badly.  I put the CO2 in and can't even get one shot out of it before the gas escapes.  It had been sitting for about 6 weeks since its last successful shooting session and I hadn't touched it in that time frame.  I located the leaks to the underside of the gas chamber.  There are two, one is on the screw that screws into the chamber and the other is due to a poor welding job on the stock screw where it comes through the gas reservoir.  My biggest complaint is that I've only had the gun for 8 months and there are maybe 1500 pellets through it.  I'm going to give Stephen Archer a call in the morning and see what he has to say about the problem.  Any ideas you folks have would be greatly appreciated.

Tyler
\"Mess with the best, die like the rest.\"

Offline ribbonstone

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RE: QB-78 problems
« Reply #1 on: March 09, 2010, 10:03:40 AM »


It may be a very simple fix.

Are three places a Qb can leak.  If the end cap 0-ring is bad, it leaks at the end cap.  If the valve stem is bad, it leaks out the muzzle (when the bolt is closed).


Yours sounds like the 3rd possible leak, the 0-ring that fits around the middle of the valve itself, sealing it to the tube.  The valve is in two parts, with the o-ring in the middle.  Idea is that when the vale is in place, you tighten it, and that forces the o-ring out to seal the gas tube.  

Might be that that o-ring is leaking...or it might be that the valve wasn't tightened enough to force the o-ring to seal.

I'll go look for some pictures.
Robert

Offline ribbonstone

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RE: QB-78 problems (pictures might help)
« Reply #2 on: March 09, 2010, 10:19:01 AM »
Ok…this is where the valve sits in the gun.  


Below the rifle, there are 3 tools that can reach in that far to turn the valve (compressing the o-ring that fits between the brass and Al. parts of the valve).  One is just a long screw driver…the other is long length of flat stock….and the 3rd is a screw driver tip cut onto a length of Al. rod.



With the end cap off, lookind down the tube, can see the slot across the Al. section of the valve.  As you tighten ths, it forces the o-ring in the middle to contact and seal the valve to the gas tube.




I’d do this:
Take off the end cap and try to tighten the valve body (top picture).  Don’t go all postal on it, but if you can turn it…it might seal…and you may not need to take it all part to replace that o-ring.


Robert

Offline geewhiz380

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Re: QB-78 problems
« Reply #3 on: March 10, 2010, 04:37:51 AM »
could just be the seals

Offline Tpatner412

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Re: QB-78 problems
« Reply #4 on: March 10, 2010, 10:01:57 PM »
I called Archer Airguns today and the lady I spoke with told me that the problem is most likely the valve stem.  So I went and looked it up and apparently replacing the stem requires complete dissassembly of the gun which I am not very happy about.  the only good thing is that the part only costs $10 or so.  Any ideas how to replace this?
\"Mess with the best, die like the rest.\"

Offline Bentong

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Re: QB-78 problems
« Reply #5 on: March 10, 2010, 11:08:46 PM »
Try here, shows and tell how to break it down  >> http://www.gatewaytoairguns.com/library/Charlie%E2%80%99s%20QB-78%20Tune.htm