problems with QB78 in 177 -massive leaking

problems with QB78 in 177 -massive leaking

Joined: June 21st, 2006, 10:40 am

June 16th, 2012, 9:17 pm #1

just bought a second hand and first time i changed carts all the gas comes out the breech and barrel and after 30-40 sec gun is empty - now wasted 8 carts and already loosing interest (again) in CO2. Just bad luck and I am not blaming the seller . I have in detailed followed all instructions I am am 100 percent convinced that the gun needs resealing (i am thinking all seals incl valve) . I have resealed my benji pumper without problems but never a co2 gun . I have read many of the older posts here on the QB78 and heard of Melick and Archer and so on - seen Archer on youtube but really not much on the who;e reseal unless you buy the $27 repair book ? . I am really thinking making it into a bulk co2 (but not too bulky) or pcp but then again dont want to spend too much. Dont have pcp gear anymore though. What would you do ? sorry for my rant but I am starting to think I am done with co2 even though this one was really for my kid to use . Thanks !
"somebody has to take the high road..."
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Joined: December 18th, 2009, 3:35 pm

June 16th, 2012, 11:01 pm #2

-Could be dirt in the valve. Search for archers pellet gun oil and firing sequence, it might help

-Is the bolt closed when you are loading the cartridges? If someone shimmed the hammer spring, or has a strong one in there you can have a similar issue so leave the bolt open when you load the c02 and pressure the co2 tube.

-If neither of those are an issue I would get a new valve stem and replace it possibly the seal is toast. The pellet gun treatment in the first point might help this, might not... but the oil will help the seal swell if you needs the valve stem replaced.

-The filter in the valve could have blown apart and a piece holding the valve open.

-You can test the gun with only 1 co2 cartridge. To do this put an empty one in the gun like you woudl normally, Then put the full one in towards the cap. Screw down the cap till is pierces the cartridge and then back the cap out part till you start to hear the co2 come out of the cartridge and fill the co2 chamber

This is what the valve stem looks like



Qb's are very easy to work on. FYI you need a special wrench to remove the valve from the tube.

Good luck,
Bruce
Last edited by bruce.itterman on June 16th, 2012, 11:03 pm, edited 1 time in total.
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Joined: June 21st, 2006, 10:40 am

June 16th, 2012, 11:36 pm #3

did load with open bolt and tried the pell oil , neither makes any dif . now after wasting many carts i do test with one empty and one full but this is getting expensive . older youtube from compacsseco (techforce 78) talks about the seal in the main valve and another seal that might be worth while replacing - actually very good movies from Compasseco . just a little disappointed so sorry for my impatience .........
"somebody has to take the high road..."
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Joined: January 9th, 2005, 1:46 am

June 16th, 2012, 11:47 pm #4

just bought a second hand and first time i changed carts all the gas comes out the breech and barrel and after 30-40 sec gun is empty - now wasted 8 carts and already loosing interest (again) in CO2. Just bad luck and I am not blaming the seller . I have in detailed followed all instructions I am am 100 percent convinced that the gun needs resealing (i am thinking all seals incl valve) . I have resealed my benji pumper without problems but never a co2 gun . I have read many of the older posts here on the QB78 and heard of Melick and Archer and so on - seen Archer on youtube but really not much on the who;e reseal unless you buy the $27 repair book ? . I am really thinking making it into a bulk co2 (but not too bulky) or pcp but then again dont want to spend too much. Dont have pcp gear anymore though. What would you do ? sorry for my rant but I am starting to think I am done with co2 even though this one was really for my kid to use . Thanks !
I agree with everything he says.

It does sound as if the valve stem seal is bad. To replace this, the valve needs to be removed. Here's the "Cliff Notes" version of how to do this...

Following on from my YouTube videos, simply remove the trigger assembly by undoing the front screw then slide the hammer assembly back out of the main tube.

The only real issue is how to remove the valve itself. The valve body first needs to be loosened, this can be done by looking down the main tube from the tube cap end and inserting a large screwdriver (or similar tool) into the slot. Give a turn anti-clockwise to loosen the valve body and relax pressure on the O ring seal around the valve. After you have done this, remove the small Philips setscrew that holds the valve in place - it's in the underside of the gas tube. Once the valve is loosened and the setscrew removed, I use a length of dowel to drive the valve forward out of the tube (don't drive it out of the back as the rough edges of the slots and holes in the tube are certain to damage the valve O ring).

That's it! unscrew the two parts of the valve body and replace the valve stem assembly. Reassemble in reverse order. And yes, you'll probably find that it's worthwhile to replace all the O rings and seals as you rebuild the gun.

Converting the gun to CO2 bulk fill is simple, but the parts are not inexpensive. You can see an example of this in our QB78 Bulk Fill Kit http://www.archerairguns.com/QB78-Bulk- ... /aabfk.htm

But you can gain an increase of 40-50% in the number of shots per fill by fitting an HDD and continuing to use Powerlets. There's a post just a little down the page about this. It pays for itself in very little time. You can see the product itself at http://www.archerairguns.com/QB78-HDD-p/qb78hdd.htm

HPA conversions require more cost, including probably a new main tube to withstand operation at higher pressures than CO2. If you don't know much about the previous history of your QB78, I'd be VERY cautious before moving down that route for safety reasons.

I hope this helps! Don't give up on CO2 - at least not yet

Stephen Archer
Archer Airguns Inc.
Online stores http://www.archerairguns.com
http://www.archerpelletguns.com
Videos http://www.youtube.com/ArcherAirguns
Blog http://www.archerairguns.info

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Joined: June 21st, 2006, 10:40 am

June 17th, 2012, 1:31 am #5

i appreciate the advise , not sure if i want to stick with it though . thx again
"somebody has to take the high road..."
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Joined: September 21st, 2007, 12:13 pm

June 17th, 2012, 2:15 am #6

did load with open bolt and tried the pell oil , neither makes any dif . now after wasting many carts i do test with one empty and one full but this is getting expensive . older youtube from compacsseco (techforce 78) talks about the seal in the main valve and another seal that might be worth while replacing - actually very good movies from Compasseco . just a little disappointed so sorry for my impatience .........
put only one full 12g into the gun, it can be in either location, however I usually prefer using the full one in the position against the valve and the empty toward the muzzle. Cock the gun (no pellet in the breech)and fire it repeatedly as fast as possible until the 12 runs dry. In doing this, on several occasions I've successfully cleared debris from under a leaking valve stem.
If no success, you'll need a new valve stem and o ring.
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Joined: June 21st, 2006, 10:40 am

June 17th, 2012, 2:27 am #7

thx will try when i get mmore time have now packed it down and will look at it after the summer . just read about 20-30 older (some not so old) post on verious issues related to QB78 and I have to say that I find a lot of inconsisten info ....sorry for saying this so harsh . one post promotes the HDD for more and harder shots then a little later report that obvious HDD is giving less shots due to its increasing velocity .... i need to give it a log rest else look later on at classified trade for a nice light and mild springer ..thanks again
"somebody has to take the high road..."
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Joined: September 21st, 2009, 3:28 am

June 17th, 2012, 3:02 am #8

just bought a second hand and first time i changed carts all the gas comes out the breech and barrel and after 30-40 sec gun is empty - now wasted 8 carts and already loosing interest (again) in CO2. Just bad luck and I am not blaming the seller . I have in detailed followed all instructions I am am 100 percent convinced that the gun needs resealing (i am thinking all seals incl valve) . I have resealed my benji pumper without problems but never a co2 gun . I have read many of the older posts here on the QB78 and heard of Melick and Archer and so on - seen Archer on youtube but really not much on the who;e reseal unless you buy the $27 repair book ? . I am really thinking making it into a bulk co2 (but not too bulky) or pcp but then again dont want to spend too much. Dont have pcp gear anymore though. What would you do ? sorry for my rant but I am starting to think I am done with co2 even though this one was really for my kid to use . Thanks !
at Flyingdragon.He will fix it and tune it,I personally don't trust any one else with my QB.
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Joined: June 21st, 2006, 10:40 am

June 17th, 2012, 4:06 pm #9

will call him in august , perhaps i only need to send the action and not the whole thing

"somebody has to take the high road..."
"somebody has to take the high road..."
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Joined: August 20th, 2006, 5:36 am

June 17th, 2012, 5:21 pm #10

just bought a second hand and first time i changed carts all the gas comes out the breech and barrel and after 30-40 sec gun is empty - now wasted 8 carts and already loosing interest (again) in CO2. Just bad luck and I am not blaming the seller . I have in detailed followed all instructions I am am 100 percent convinced that the gun needs resealing (i am thinking all seals incl valve) . I have resealed my benji pumper without problems but never a co2 gun . I have read many of the older posts here on the QB78 and heard of Melick and Archer and so on - seen Archer on youtube but really not much on the who;e reseal unless you buy the $27 repair book ? . I am really thinking making it into a bulk co2 (but not too bulky) or pcp but then again dont want to spend too much. Dont have pcp gear anymore though. What would you do ? sorry for my rant but I am starting to think I am done with co2 even though this one was really for my kid to use . Thanks !
With the QB, I have an XS60C I'll swap you for it. It's a .22, but if doesn't leak, and might be an even better choice for a kid than the QB. Email me if you're interested.

On the other hand, if you decide to get into it yourself, I have orings and extra valve stems on hand, I'd be happy to send you what you need, no charge. Later.

Dave
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