Can't Fill My 78

Hi Guys

I can't seem to fill my QB78 and have wasted at least a dozen carts trying to do it. I found that the O ring in the fill cap was perished so I replaced it but now I am having the same problem. I put in a new pair of CO2 carts and screw up the end cap, then I back it off half a turn like it says in the manual. As soon as I cock it and fire it all of the gas leaks out of the filling cap.

Any ideas?

Jon

Both of my QBs are tank models, so I can't help from experience.  However, the QB family manual that came with my rifles doesn't say how much to back off the cap.  It says to unscrew until you feel some resistance.  This good online manual says to back off a quarter turn after piercing the cartridge.  Could it be that you are just backing off too much?  Since it doesn't leak until you fire the gun to pierce the rear cartridge, could it be that you are not initially screwing the cap in far enough?  Are you piercing the forward cartridge?
http://www.foxlang.com/PDF%20files/TF79manual.pdf

Thanks for the reply, I am doing it up as far as I can with my fingers then backing it off a little. I think I will get in the workshop this week and make a bulk fill adaptor.

Jon

[DanoInTx]

I tighten mine fairly tight, then back off 1/4 turn or so till I hear the slight hiss.  Never dumped a cartridge that way.  Your problem sounds more like a leaky valve stem, or an over sprung hammer valve or really undersprung valve return spring.  Have you perhaps over shimmed the hammer spring?  Easy way to tell what's going on is where is the air coming out?  If it's coming out of the cap then obviously there is something wrong with the o-ring there, if it comes out the barrel, then it's a problem with the valve not closing tight.  IF it sounds like the air is escaping from somewhere else, it might be the oring around the valve that seals it inside the air chamber.  Other than those three places, the only other seals as I recall are the transfer port, barrel and the bolt breach o-ring....none of these will cause you to dump air like that, just lose power.  Also, normal hardware store plumbing seals don't work for the cap and valve orings, the material absorbs CO2 and makes them swell then leak...I think someone here mentioned Viton o-rings here, not buna.  If it's the knock open valve seal (which it sorta sounds like), you might just try dissassembling and looking for dirt or whatever under the sealing face.  Have you been wiping your cartridges off before installation?  A little dirt on a caplet will gum that valve up pretty fast.  Good luck,

Dan

The debate between a quarter and a half-turn backoff is without meaning. If all of your seals are good, you will only be able to back the gas tube cap off a fraction of a turn before it is stopped by gas pressure. This is the simplest test of all seals, and you only need to waste ONE cartridge to make it. Load an empty powerlet FIRST, followed by a new cartridge. Screw the cap on and back it off. If the cap doesn't stop of its own accord, gas is leaking out of the gas tube - either through the cap or through (or around) the valve.

You mention the cap seal went bad and was replaced. If it failed for chemical reasons (like lubricating the rifle with petroleum-based lubricants or bore cleaner), it is likely ALL of the seals are bad. If it failed mechanically (such as tearing on a burr), you may be luckier.

Did you check to see if the valve is loose in the gas tube? (A 1/8 x 3/4 x 8" steal bar makes a great valve  wrench.)

If all else fails, I'd pack it up and send it to Mike Melick (mikeiniowa). Send Mike a message through his site:http://www.airgunartisans.com/flyingdragon/.


Good luck!

George

What is the problem with lubricating with petroleum based lubricants.  Non-detergent motor oil (20 or 30 weight) is what is used to lube the seals, in moderation of course.  The buna and Viton seals are resistant to motor oil.

Hi Guys

The 78 is a standard one that hasn't been touched since new. I will strip it down this week and have a look but I am getting sick of using the gas carts so I am going to make a bulk fill for it this week. I have an Air Arms S200 air cylinder here that I might use the fill valve from, I am not going to use the whole cylinder as it is so heavy and my son is only ten and it's his gun. I have some thick walled aluminium tube so I might make a completely new longer cylinder for it. It doesn't work well in the cold weather anyway so I might as well work on it during the winter.

Jon

[ribbonstone]

Make sure it's the cap that's leaking, and not gas rushing out of the barrrel....waste one more cartridge, covering the muzzle with your finger to see if that's the leak.

If it's the muzzle, then something has fouled your exhaust valve.  Have had that happen when the fiber filter sheded fibers, some of which were caught in the valve.

IF it's the end cap, and you've replaced the o-ring,  then something is cutting the o-ring as you screw it on...a burr just has to nick it a little bit to ruin it.

I fixed my 78, there was dirt on the filler cap thread stopping me screwing it up tight enough. This meant that it wasn't puncturing the forward cart then it leaked when I fired it and the rear cart was punctured.

Jon

[mikeiniowa]

Thank heaven it was something simple

I still need pliers to do it up though!

Jon

[mikeiniowa]

Jon, are the threads on the tube buggered or the cap, I can send a new tube or cap or both if you need them, shipping to you isn't that bad. Mike

[IJL]

Jon, looks like you may have to send that one to me as well Let me know if I can help, I'm a little closer than Mike!

Ian

Hi Guys

Thanks for the offer Mike but I am planning on HPA converting it sometime so will do away with the end of the tube in order to fit a bottle. IJL and I are working on plans at the moment, I'll get it to you soon Ian.

Thanks

Jon

[Gene_SC]

Here is what I have found... My two cents worth  hehe

I had the same problem. The o'ring on the fill bottle would disinagrate. Gun would leak after I change the seal. It was pieces of seal getting inside the gun and loging in the valve and seat. Very small. I never did find a solution. But made sure my o'ring was good before I filled any of my bulk QB's again. It helps to smear a little silecone 30 weight oil on the o'ring before each fill..

Gene

PS, Roy from Mountain air could give you professional advice on this problem. Send him an email. He is good about answering his emails..