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Tuning an SMK QB78 problems!

Discussion in 'Anything Airgun Related' started by Frenchie, Dec 3, 2008.

  1. Frenchie

    Frenchie Engaging Member

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    Location:
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    I bought a brand new SMK QB78 from Manchester Air Guns with the intention of using it for close range pigeon culling knowing that T R Robbs sold a "Tuning Kit" which they say on their instruction video will take that gun up to 11 - 11.9 FPE. I have just fitted a full "Tuning Kit" (which cost £56.00) and after watching the instructional video twice (Exactly as they recommend). I carefully fitted each part first making sure that every part was clean and free from any burrs or blemishes. I used the standard hammer spring (just as they say you must do) and tried the gun. The chorno at Bury Gun Rooms said it was giving 7.56 FPE (No improvment what so ever) so I took it home again and tried one of the sllightly larger hammer springs and tried again on the chrono - 7.8 FPE. Back home and strip the gun again, this time drilling the 2 extra 2.5mm holes in the Piercer Pin Cap (which was supplied with the kit and marked for drilling to give better gas flow - as per their video). Tried the gun and took it back yet again to Bury Gun Rooms to test on the chronograph - all it showed was a consistent 8.5 FPE (512 - 517 FPS)
    This power increase is a long way short of what T R Robbs are claiming on their web site and instructional video where they claim this tuning kit will take the gun up to 11 - 11.9 FPE so has anyone else tried one of these tuning kits and what results have they achieved please. If anyone knows what the problem might be I would be grateful if they could let me know before I phone T R Robbs tomorrow (They are closed all day today) to see what they can do about me wasting my time and money on this "Tuning Kit" that has made practically no difference at all except to my credit card.
     
  2. dickyd

    dickyd Engaging Member

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    i hate to say it but yours must be the exeption to the rule,my 78 was doing approx 9.5fl/lb untuned,fitted new probe and valve body,and now running at just over 11ft/lb,so i would definatley have a word with t r robb,and pick his brains,he sholud be able to help:)
     
  3. dlloyduk

    dlloyduk Well-Known Member

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    Have you checked that the transfer port is lined up correctly from what I hear can make a big difference-but as above talk to the experts. From what I,ve been reading about these 78,s recently (a lot) you should have more trouble staying within the limit rather than manage to make it. Good luck I,m watching with interest.
     
  4. Frenchie

    Frenchie Engaging Member

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    Location:
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    Yes, I have checked that the transfer port and port seal are all ligning up with the barrel and just noticed that thePort through the bolt housing was not ligning up properly with the port in the barrel, this seems like it might be restricting the gas flow. I have now machined the barrel port and it looks a lot better. On reasembling the gun and trying it the gun certainly seems to have a lot more "poke". On testing the gun on an old "Yellow Pages" it sends the pellet nearly twice as deep as it did before, so it looks like I might be getting there. All that remains to do is to get back to Bury Gun Rooms and have it tested on the Chronograph, but with the weather we have at the moment (About 8 inches of snow on the ground at the moment and more forecast) I am not going to try to get to Bury till the roads are a lot clearer.
    Hopefully all's well that ends well!
     
    Last edited: Dec 4, 2008
  5. i8allthepies

    i8allthepies Posting Addict

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    Hi Frenchie,
    Just a reminder that the ambient temperature has a BIG effect on these guns:up:
     
  6. Frenchie

    Frenchie Engaging Member

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    Location:
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    Hi Davy, Yes I am aware that the ambient temperature can have a major effect opn CO2 guns, but thanks for pointing that out. I have done all my tuning and testing in a room with the ambient temperature around constant all the time. As I said in my previous post, I have got the gun giving much better power now (the difference is phenomenal) so I need to go to Bury Gun Rooms to have it chronographed to ensure that I am still within the legal limit. I will give an update with the chronograph reading as soon as poss.
     
  7. dlloyduk

    dlloyduk Well-Known Member

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    Did you find out what the problem was?
     
  8. Frenchie

    Frenchie Engaging Member

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    Location:
    Bacup, Lancashire.
    Yes I did find the problem, it was the least expected thing. The port on the barrel/breach was not ligning up with the other gas ports. To rectify the problem I had to machine the port that runs through the bottom of the barrel/breach on a bit of an angle to get the gas flowing properly. Now that I have done that the gun is putting out a great deal more power - so much so that I need to take it and put it on a chronograph to make sure that I am still within the legal limit. The way that the gun is performing at the moment I suspect that it is very close to the legal limit, but the chrono will tell me exactly what power it is putting out once the weather allows me to drive safely to Bury Gun Rooms and test it. I will post the readings as soon as poss!
    Thanks for the help and advice.
     
  9. i8allthepies

    i8allthepies Posting Addict

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    Yay! Glad it worked out:up:
     
  10. oldhunter

    oldhunter Newbie

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    You can also drill out the oval hole in the bolt probe and the passage threw the end of the probe, using a 2.5mm drill. This will increase flow without changing the number of shots you get from a fill. I have also put a washer behind the hammer spring I get 9.2 f/lb without a modified valve.
     
  11. JK1

    JK1 Newbie

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    SMK QB78 Deluxe Tuning - Easy/Hard

    Hi There,

    I have bought a SMK QB78 Deluxe and i am wanting to tune it using T R Robbs tuning kit.

    I am reasonably good with my hands (so to speak) but i have never taken apart an air rifle before. Im wondering how hard it would be to tune with TR Robbs kit and if T R Robbs' demo video is easy enough to follow.

    Many thanks
    JK
     
  12. mick 1986

    mick 1986 Keyboard Hero

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    dont waste your money on the TRobb gear mate, just mod the standard parts. if you can strip the rifle then its as easy as pie, by the way, anyone can strip these rifles with the correct guide!

    take a look at the ukchineseairgunforum.

    mick.
     
  13. JK1

    JK1 Newbie

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    Thanks Mick,

    I check that out,.
     
  14. jantar

    jantar Donator

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    Bejabbers....I wish I had read this post first cos we have just returned from Leicesrshire Airguns after buying one (the Deluxe) in .22 for 129 notes for my wife (newbie) to start accompanying me to the range.
    Must say that when I unpacked it the build quality was excellent but lousy sticky safety label took some getting off the forestock! Once I had eventually got rid of and cleaned the gunk off it left behind I loaded the CO2 and test fired it using the standard sights. It was way more accurate and quiet than I had expected for the money so I thought 'wahat a great buy'.
    Then I got on the net and looked at this post after bookmarking the Robb site so I could return and buy the tuning kit they offer with the statement that it will almost double the power to just under the legal limit.
    Can someone tell me if by installing the kit correctly ( I am a retired precision engineer) the kit will do as actually do they state. OR will it only acheive a very modest increase for a lot of money?
    Out of the 8 different (all quality ones) test pellets I have used it was the Milbro TR (twin ring) which gave the best group and accuracy straight out of the box so they are what I will stick to.
    I have a chrono and I shall run it past it later today because I keep a monthly record of the power output of all of my guns so I can stay within the law. I will post again once I have taken the readings.
    Being honest I might even buy one for myself now cos I do like the way it handles............(how can I justify adding another to my collection without parting with any of the others?)
     
  15. vitalich

    vitalich Keyboard Hero

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  16. jantar

    jantar Donator

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    OK...I will give it a go and take extra care with the build. Last thing I want is to become illegal..........:up:
     
  17. secretagentmole

    secretagentmole Low down, dirty and quiet...

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    I bought a QB78 DL in .22 for tuning and playing about with. I got the T R Robb kit and did the full part swap, the gun was 14 ft lb after that!!!

    The only way I got it back down was to put the original hammer block back in. That got it down to 11.6 ft lb!

    It is an easy to use tuning kit and video. Does what it says on the tin, has a great effect on the power too, mine was just over 9 ft lb before I tuned it!
     
  18. jantar

    jantar Donator

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    Very nice that it worked for you but all I had are problems with the full .177 tune up kit. With the kit I was supplied the bolt would not fully close into its locked position at all yet the original parts did! Being fair though Terry (Robb) was called and after speaking and a few emails I was asked to send back the modified bolts he had supplied plus the original barrel. I also ordered a new modified bolt as a spare just in case.
    To give him the best chance of resolving the problem I also enclosed the original breech block, original bolt and handle. That was a week ago and I am now hoping that I will soon receive them back.
    When it arrives I will then be able to see what happens when I try in the spare modified bolt I had ordered in my new XS79.
    What I cannot understand is why his .177 kit did not work in my .177 XS79 when all of the .177 components are the same as the XS78?
    Any ideas??
     
  19. mark1967

    mark1967 Engaging Member

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    Good advice ^
    Very helpful bunch and a mine of information
     
  20. besageezer

    besageezer Well-Known Member

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    In my experience in tuning the 78 or 79, the best bolt probe modification is to cut off the 'tube', drill out the end of the bolt to accept a piece of 3/32" silver steel rod held in place with Loctite but long enough to push the pellet past the transfer port. Around 100fps gain.
    There is a great deal of info on Charlie Da Tuna website on how to tune up the QB but be mindful that it can easily take it over the limit.
    Lining the transfer port up with the valve port can be a problem, the valve fixing screw can pull it out of position due to poor engineering tolerance and the hole in the bottom of the main tube may need modification to allow the transfer port to align. If the transfer port does not line up with the barrel port there will be a problem with sealing between the two. I found a piece of soft clear plastic tube around a 1/4"ID helped to solve the issue. Sprinkle some talc around the seal, fire it and you will see if the seal is good.
     

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