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Advice QB 78DL

Discussion in 'Anything Airgun Related' started by sosmart2k3, Apr 8, 2014.

  1. sosmart2k3

    sosmart2k3 Member

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    Location:
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    Hi everyone
    PLEASE HELP ME !!!
    Is there anyone in the Bristol area that could look at my QB78 ?
    I have had it apart and changed seals ,changed the probe ,cleaned ,de burred and polished all the internal organs as per a download I got from here,
    It worked ok ish when put back together ,it was put back in the cupboard ,went to use it today and surprise surprise the gas had gone,put new gas in and it came straight out .
    I am at the end of my tether with this gun ,as far as I can see there is nothing obviously wrong with it ,but it will not work consistently.
    Its getting close to going in the bin!!!
    Any help or advice would be gratefully accepted
     
  2. djwalker

    djwalker Donator

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    Location:
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    were does the gas come out of , breach , barrel.?. did you re tighten the valve body , what mods have been done.
     
  3. sosmart2k3

    sosmart2k3 Member

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    the gas seems to come from everywhere
    this isnt the first time i have had problems with it but i cant seem to figure this one out
    as for the mods done ,I bought a modified probe from Archer airguns online ,all interior surfaces de burred and polished new seals ,the interior parts of the valve removed (gauze, filter etc)
    no new springs or anything else
     
  4. owain glyndwr

    owain glyndwr Posting Addict

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    Location:
    angelesy
    it might be the valve stem not sitting properly
     
  5. sosmart2k3

    sosmart2k3 Member

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    what would cause this to happen ?
    I tried to make sure everything was really clean when I put it back together
     
  6. gunsnlandies

    gunsnlandies Posting Addict

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    Where did the seals come from, and were they coated in silicon. Did the washer and valve guide get refitted properly. Sometimes the washer can sit at a slight angle to the bottom so the guide doesn't go in far enough. But far enough to allow the body to screw on partially. It also doesn't need over tightening, so as to over distort the seal. Has the TP seated properly. Did you over polish the tube as this can be problematical. It has been known. If it was put away with a low amount of pressure and the springs (hammer and valve) are wrongly tensioned, it can let the gas go. Had this with my AR2079A a while back. Had to rework the balance. This only happened after it had been tuned (valve tapered etc). Straight out of the box, it worked fine. Had no problems with my tuned TH78 at all. If the seals are de-pressured and left, then recharged, they do not expand quick enough before the gas is expelled.
    If all else fails, send it my way instead of the bin, most people reckon I talk rubbish:D
    Brian
     
  7. sosmart2k3

    sosmart2k3 Member

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    the seals came from welsh willy .
    I'm pretty sure that everything went back straight and not over tightened
    really starting to p**s me of now as its so un reliable
     
  8. gunsnlandies

    gunsnlandies Posting Addict

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    Try putting it back together with all the originals parts and re-coating the seals from the WW bag they came in. Use one empty and one full co2 capsule so as not to waste double the gas. Lastly, did you check the height of the TP seal before fitting the new one. These sometimes need rubbing down on a bit of wet-n-dry to shorten them.
    Brian
     
  9. Lanky94

    Lanky94 Engaging Member

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    Location:
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    Does it leak when fired, or all the time?


    All the time

    1. bit of crap in between your valve face and the soft rubber seal under the valve head. Check the rubber seal
    2. Goofed valve o ring, or simply not tighted down

    or.....
    When fired.

    As said before.....too longer, or too short transfer port
     
  10. jantar

    jantar Donator

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    Location:
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    First question has to be "Did it leak before it was tampered with"? If not then obviously it has been caused by either bad or incorrect parts being used or incorrect assembly. Cheap weapons are cheap for one reason and that is because they do not have the tighter tolerance controls on specifications that the more expensive ones do. I have now modified 2 of these and I am on my 3rd one now. None have suffered leaks other than when I used a multifill adapter made to fit one manufactured by a different reservoir manufacturer whose bleed hole was 10mm from the end instead of 5! (The QB78 is the only model I know to have adopted this practice) and probably because it is made under license. I rectified that problem with the addition of an extra O rings and it has not leaked or lost air since. Yes it was an HPA (high pressure air) conversion too.
    Might I suggest that instead of giving up on it you revert it back to its original form and then do one step at a time with your upgrades, testing each as you go until you either discover the problem or it is cured of its leak/loss.
    Please don't give up now because this is a very good rifle and is loads of fun to shoot as many at my local range who have 'borrowed it' have commented even though they will not admit they might buy one! Prestige is all in this sport which is why I love my Daystates................too.
    Worst case.....you could chuck it to welshwilly and let him do his magic..........
     
    Last edited: Apr 9, 2014
  11. sosmart2k3

    sosmart2k3 Member

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    yes i agree with you
    I think the only way is to change it bit by bit ,its a shame there is no way of testing the individual components under pressure ,its costing me a bloody fortune in gas though :(
     
  12. ped

    ped Big Poster

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    try putting a balloon over the muzzle secured with a tight elastic band and leave overnight with the breech shut if the balloon is inflated then its the valve stem seal
    overwise its either the endcap seal or the valve seal what i do is strip the gun of it's trigger breech and barrel (don't take the stock stud out )and put it gas plug first into a bucket of water if that doesn't show any bubbles i submerge it further till the valve exhaust is submerged, this will show up any leaks
    ped
     
  13. sosmart2k3

    sosmart2k3 Member

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    ped
    that is the most stupidly simple way to test for leaks !! why the hell didn't i think of that .
    thanks for that, I cant believe that something so obvious could possibly save so much time .
     

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