QB78 Carbine: Degassing and clogging?

QB78 Carbine: Degassing and clogging?

Joined: July 2nd, 2001, 12:16 pm

December 1st, 2010, 8:31 pm #1

<p>Ok, so I replaced my barrel seals, my barrel set screw and shimmed my barrel band to tighten everything up and improve groupings.</p><p>I took off the loose muzzle break and tested 18 different pellets.  THANKFULLY the RWS Meisterkugelns produce a one-hole, 3-shot group at 10 yards (and a 618 fps).</p><p>After shooting some 40 pellets, in the testing, I decided to use up the rest of the gas by shooting more pellets.  Shot 5 pellets, each one getting slower and slower and the report getting dimmer and dimmer.  Finally it just stopped.</p><p>I began to unscrew the cap to swap out the carts and encountered resistance.  Not a lot, but certainly there was pressure in there.  I dry fired the rifle to release the gas.  The hammer fell, but no gas came out the barrel.  Continued to unscrew the cap with the same resistance.  Dry-fired to release gas...still no gas from the barrel.  Finally got to the last thread and had to pull the cap/plug out.  The plug came out, the two spent carts went shooting into the wall.  There was still pressure in there that was NOT released by either the dry firing or the bleed feature of the cap.</p><p>What the hell!  Is my transfer port clogged?  Why didn't the cap release the gas before coming unthreaded like on the Crosman 600?  It looks like the same design.</p><p>Now I have a rifle with a pellet in the chamber that did not have any gas flow on the last two shots.  I don't want to gas it up again for fear of never getting the cap back off.  Anyway to check for a transfer port clog without totally dismantling the thing or gassing it up?</p><p>Kris in NY</p>
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Joined: August 16th, 2006, 10:39 pm

December 1st, 2010, 8:49 pm #2

if that don't work a wood dowel to push it back toward breech....and did a pellet get lost recently and fall into the action stopping the hammer travel??? I cant see the transfer port clogging all the way up. Did the valve have the infamous filter and screen still in it???
Last edited by critter99 on December 1st, 2010, 8:51 pm, edited 1 time in total.
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Joined: July 2nd, 2001, 12:16 pm

December 1st, 2010, 9:15 pm #3

<p>...but wondering how to find the clog without taking it all apart. </p><p>Thanks for the reply.  </p><p>Don't know about filter or screen.  Sold to me as a "Melick-tuned" rifle.  Can't find out what Mike does to his products.  Do you have a link to some discussion about the infamous screen and filter so I can see what sort of trouble I am in?</p><p>Kris in NY</p>
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Joined: August 16th, 2006, 10:39 pm

December 2nd, 2010, 1:47 am #4

he removes them if he open it up I will look for thread on them. Maybe it was not prepped by Mike mine shoots 14.3 cps at 720 FPS on Co2....check out the valve picture he was using HPA but I have heard the filter can fall apart on Co2 as well. I wonder if that is what stopped yours from dry firing out the charge???
Last edited by critter99 on December 2nd, 2010, 2:40 am, edited 1 time in total.
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Joined: August 16th, 2006, 10:39 pm

December 2nd, 2010, 1:59 am #5

<p>Ok, so I replaced my barrel seals, my barrel set screw and shimmed my barrel band to tighten everything up and improve groupings.</p><p>I took off the loose muzzle break and tested 18 different pellets.  THANKFULLY the RWS Meisterkugelns produce a one-hole, 3-shot group at 10 yards (and a 618 fps).</p><p>After shooting some 40 pellets, in the testing, I decided to use up the rest of the gas by shooting more pellets.  Shot 5 pellets, each one getting slower and slower and the report getting dimmer and dimmer.  Finally it just stopped.</p><p>I began to unscrew the cap to swap out the carts and encountered resistance.  Not a lot, but certainly there was pressure in there.  I dry fired the rifle to release the gas.  The hammer fell, but no gas came out the barrel.  Continued to unscrew the cap with the same resistance.  Dry-fired to release gas...still no gas from the barrel.  Finally got to the last thread and had to pull the cap/plug out.  The plug came out, the two spent carts went shooting into the wall.  There was still pressure in there that was NOT released by either the dry firing or the bleed feature of the cap.</p><p>What the hell!  Is my transfer port clogged?  Why didn't the cap release the gas before coming unthreaded like on the Crosman 600?  It looks like the same design.</p><p>Now I have a rifle with a pellet in the chamber that did not have any gas flow on the last two shots.  I don't want to gas it up again for fear of never getting the cap back off.  Anyway to check for a transfer port clog without totally dismantling the thing or gassing it up?</p><p>Kris in NY</p>
http://www.network54.com/Forum/113813/t ... the+QB+PCP
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Joined: March 21st, 2010, 3:13 pm

December 2nd, 2010, 4:38 am #6

<p>Ok, so I replaced my barrel seals, my barrel set screw and shimmed my barrel band to tighten everything up and improve groupings.</p><p>I took off the loose muzzle break and tested 18 different pellets.  THANKFULLY the RWS Meisterkugelns produce a one-hole, 3-shot group at 10 yards (and a 618 fps).</p><p>After shooting some 40 pellets, in the testing, I decided to use up the rest of the gas by shooting more pellets.  Shot 5 pellets, each one getting slower and slower and the report getting dimmer and dimmer.  Finally it just stopped.</p><p>I began to unscrew the cap to swap out the carts and encountered resistance.  Not a lot, but certainly there was pressure in there.  I dry fired the rifle to release the gas.  The hammer fell, but no gas came out the barrel.  Continued to unscrew the cap with the same resistance.  Dry-fired to release gas...still no gas from the barrel.  Finally got to the last thread and had to pull the cap/plug out.  The plug came out, the two spent carts went shooting into the wall.  There was still pressure in there that was NOT released by either the dry firing or the bleed feature of the cap.</p><p>What the hell!  Is my transfer port clogged?  Why didn't the cap release the gas before coming unthreaded like on the Crosman 600?  It looks like the same design.</p><p>Now I have a rifle with a pellet in the chamber that did not have any gas flow on the last two shots.  I don't want to gas it up again for fear of never getting the cap back off.  Anyway to check for a transfer port clog without totally dismantling the thing or gassing it up?</p><p>Kris in NY</p>
I had a valve stem push through and would not let it degas. I thought it was valve lock up at first. But after I got the cap off and got the valve apart the valve stem was to short to fire it. I got a new one from Archer air and have not had any trouble with it.
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Joined: September 21st, 2007, 12:13 pm

December 2nd, 2010, 4:39 am #7

<p>Ok, so I replaced my barrel seals, my barrel set screw and shimmed my barrel band to tighten everything up and improve groupings.</p><p>I took off the loose muzzle break and tested 18 different pellets.  THANKFULLY the RWS Meisterkugelns produce a one-hole, 3-shot group at 10 yards (and a 618 fps).</p><p>After shooting some 40 pellets, in the testing, I decided to use up the rest of the gas by shooting more pellets.  Shot 5 pellets, each one getting slower and slower and the report getting dimmer and dimmer.  Finally it just stopped.</p><p>I began to unscrew the cap to swap out the carts and encountered resistance.  Not a lot, but certainly there was pressure in there.  I dry fired the rifle to release the gas.  The hammer fell, but no gas came out the barrel.  Continued to unscrew the cap with the same resistance.  Dry-fired to release gas...still no gas from the barrel.  Finally got to the last thread and had to pull the cap/plug out.  The plug came out, the two spent carts went shooting into the wall.  There was still pressure in there that was NOT released by either the dry firing or the bleed feature of the cap.</p><p>What the hell!  Is my transfer port clogged?  Why didn't the cap release the gas before coming unthreaded like on the Crosman 600?  It looks like the same design.</p><p>Now I have a rifle with a pellet in the chamber that did not have any gas flow on the last two shots.  I don't want to gas it up again for fear of never getting the cap back off.  Anyway to check for a transfer port clog without totally dismantling the thing or gassing it up?</p><p>Kris in NY</p>
problem with a OEM QB78.
As it turned out in my case, the valve head had started to separate from the stem letting it shift to the rear so close to the striker that it couldn't open the valve on impact.
The fix was to replace the valve stem with one made from teflon (delrin and nylon 66 also work well) that did not have a "threw" hole that would allow the stem shaft to be blown threw the valve head rearward toward the striker.
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Joined: July 2nd, 2001, 12:16 pm

December 2nd, 2010, 5:00 am #8

<p>Damn, people really work hard on stuff that sparks their "what if..." fire.  I just wanted to shoot pellets.</p><p>I hate to fiddle with stuff.  I'll break it down, see whats what, then toss it if it won't run with parts on hand.  Already too much work for fewer than 50 shots.  I have an old springer with countless pellets through it in the 15 years I have been shooting it.  All I ever did to it was cock it and load it shoot it.</p><p>Thank you very much for the link and the advice.</p><p>Kris in NY</p>
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Joined: October 25th, 2002, 3:46 pm

December 2nd, 2010, 7:23 am #9

doubt I'm alone.

Walter...
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Joined: July 2nd, 2001, 12:16 pm

December 2nd, 2010, 1:41 pm #10

<p>Does it refer to the topic at hand?</p><p>Kris in NY</p>
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