Air leaking down the barrel... replaced the cup seal already

short304 · 05-12-2011, 11:47 PM

So I had R015 out for a second weekend (in a row) and she was shooting beautifully at about 270fps... and then half way through the day, I had a pinch mid-game, took her off the field, broke her down, cleaned her up, put her back together, and for some reason I couldn't get her shooting back over 220 (w/ Red main, yellow valve spring)... earlier I had it cranked in about as far as I could on the velocity screw to get it shooting up to 270... anyways, when I took it apart to clean and polish it today, I put her back together and it was leaking down the barrel (or more like leaking out of the breech view-port thingy)...

My 1st instinct was that it must be the cup seal, so I replaced it. I also replaced the valve seal just to be on the safe side. It was still leaking, so I thught maybe the spring combination was causing the valve to stay open, so I went down to a Green (light-med) main spring and it stopped leaking for the first two shots, and then started leaking again...so I swapped out the main spring for the yellow (light) main spring and it stopped leaking for about 10 seconds and then started leaking again...

Is there any reason that the spring combination should even cause the valve to stay open? For now, I'll be happy to figure out why it's leaking down the barrel, but any idea about why my velocity would have changed the way it did (so suddenly - when NOTHING was changed) would also be appreciated..

The Nelson design is so straight forward, I hate getting stumped by it, but alas, I need help!

ironchef97 · 05-13-2011, 12:12 AM

#1 rule: Don't clean your gun if it doesn't need it!

Do you have debris in your valve like dirt or polishing compound in there? That could cause the leak after a few shots, since the dirt would move around and eventually get on the cupseal's seal.

Maybe you need to back your TPC all the way out and start over, it's possible the TPC is too far in to the point the threads are not engaged, and that could put pressure on the powertube's shoulder.

PB Weasel · 05-13-2011, 03:00 AM

Broken Powertube from what I have read . . . Mine did the same thing, what probably happened is that one of the three pieces of metal that are in-between the powertube inlet ports has a hairline crack in it. When this happened to R018 she would shoot great for like 4 or so shots then vent down the barrel, then seal up and shoot for a couple more, then vent totally. What you should do is take it apart again and look at the base of the powertube where the cupseal screws onto it and look very discerningly at the inlet ports, and if there is any cracks you should find them. They catch and hang up inside the valve nut causing the venting you have mentioned. Good luck short and let us know what you find.

Does you gun still have the original mongo PT in it? Mine did and the reason they break is just simply because of how mongo drills the #4 tube out to #6 specs, it is basically unavoidable. Not saying anything about the way mongo chooses to build his guns, so please don't get me wrong. If the PT is original and broken as I described you can order a Lapco #6 PT from ASP and cut that down to the required length. In-fact buy a few so you can fart around with them and do what you will, and be sure to get the #6, not the #6P.

Oh and one more thing, while you are placing an order with ASP do yourself a favor and buy a 6-32 threaded lapco nylon cupseal, they are pissa and last forever! And it is one piece so there are no parts to rebuild.

ironchef97 · 05-13-2011, 04:09 AM

that should be a #4 bored to #6 ID if none of the owners after me (

) changed the pt out.

CJOttawa · 05-13-2011, 07:29 AM

Common leak: the valve nut o-ring. The nut should be tightened down fairly tight to properly seal the o-ring and it's common that it won't seal fully.

Also: no need for a complete tear-down to clean. If you're feeding the gun clean 12-grams (MOSA or Leland) there won't be anything in the valve as pressure inside the valve blows debris OUT. (caveat: unless your gun is buried in sand or water while degassed...)

Just remove the snub (single screw!) and clean valve-forward.

Weasel: I've broken a powertube as well but that manifested as not firing at all or a huge leak followed by no gas left. Also, it would be noticeable immediately on opening the valve.

PB Weasel · 05-13-2011, 07:49 AM

What you said is very true CJ, however my PT only snapped one of the three supports, causing it to hang up and partially vent a couple times per 12ie. Of course when they fail as yours did ( snapping completely in two) the issue is much more noticeable. Good luck with righting your dux short.

WickedKlown2 · 05-13-2011, 09:04 AM

Weasel,,,

I talked to Sergey at LAPCO and he told me the difference in the LAPCO #6 and #6P and the shoulder on the 6P is stronger than the 6... The 6P also uses a undercut cupseal that seals better than the flat face cupseal for the 6...

All I use is the Lapco 6P in all my Nelson based markers and when I had my DuckSlide I cut down a Lapco 6P and put in it and never had a problem... Ask TheYoda to see it

next time you see him...

------- ----- ------- --------- --------

Short if you are coming down for the Mid-Tenn Outlaw game on the 21st of May I can cut down a Lapco #6P and Lapco Delrin Cupseal for you...

WK2

DSA · 05-13-2011, 11:12 AM

Don't forget to check the 12gm seal inside the grip frame. They routinely pop out if your short swapping cartridge.

PB Weasel · 05-13-2011, 12:45 PM

Well now WC2, thanks for the info. I was told something totally different, I shall promptly smack them in the mouth

learn something new everyday

WickedKlown2 · 05-13-2011, 03:08 PM

Weasel,,,

Anytime Bro

The reason I found out is there was a discussion one time about the difference so I asked the expert at Lapco to get the answers I was needing...

Knowing is Half the Battle

WK2

05-13-2011, 12:12 AM	#2 (permalink)
ironchef97 . Join Date: Sep 2008 Location: SD	#1 rule: Don't clean your gun if it doesn't need it! Do you have debris in your valve like dirt or polishing compound in there? That could cause the leak after a few shots, since the dirt would move around and eventually get on the cupseal's seal. Maybe you need to back your TPC all the way out and start over, it's possible the TPC is too far in to the point the threads are not engaged, and that could put pressure on the powertube's shoulder. __________________ Avratech R7 review \| DSP frame review Member's only deals:Duckslide, Carter buzzards and comps, Phantom, Karnivor pump

05-13-2011, 03:00 AM	#3 (permalink)
PB Weasel Sneaky Little Bastid #3 Join Date: Apr 2010 Location: Burlington, Massachusetts	Broken Powertube from what I have read . . . Mine did the same thing, what probably happened is that one of the three pieces of metal that are in-between the powertube inlet ports has a hairline crack in it. When this happened to R018 she would shoot great for like 4 or so shots then vent down the barrel, then seal up and shoot for a couple more, then vent totally. What you should do is take it apart again and look at the base of the powertube where the cupseal screws onto it and look very discerningly at the inlet ports, and if there is any cracks you should find them. They catch and hang up inside the valve nut causing the venting you have mentioned. Good luck short and let us know what you find. Does you gun still have the original mongo PT in it? Mine did and the reason they break is just simply because of how mongo drills the #4 tube out to #6 specs, it is basically unavoidable. Not saying anything about the way mongo chooses to build his guns, so please don't get me wrong. If the PT is original and broken as I described you can order a Lapco #6 PT from ASP and cut that down to the required length. In-fact buy a few so you can fart around with them and do what you will, and be sure to get the #6, not the #6P. Oh and one more thing, while you are placing an order with ASP do yourself a favor and buy a 6-32 threaded lapco nylon cupseal, they are pissa and last forever! And it is one piece so there are no parts to rebuild. __________________ My Feedback:http://www.mcarterbrown.com/forums/f...ml#post1449446 Last edited by PB Weasel; 05-13-2011 at 03:11 AM. Reason: more elaboration :)

05-13-2011, 04:09 AM	#4 (permalink)
ironchef97 . Join Date: Sep 2008 Location: SD	that should be a #4 bored to #6 ID if none of the owners after me () changed the pt out. __________________ Avratech R7 review \| DSP frame review Member's only deals:Duckslide, Carter buzzards and comps, Phantom, Karnivor pump

05-13-2011, 07:29 AM	#5 (permalink)
CJOttawa Immune to sales tactics. Join Date: Dec 2006 Location: Canada's National Capital	Common leak: the valve nut o-ring. The nut should be tightened down fairly tight to properly seal the o-ring and it's common that it won't seal fully. Also: no need for a complete tear-down to clean. If you're feeding the gun clean 12-grams (MOSA or Leland) there won't be anything in the valve as pressure inside the valve blows debris OUT. (caveat: unless your gun is buried in sand or water while degassed...) Just remove the snub (single screw!) and clean valve-forward. Weasel: I've broken a powertube as well but that manifested as not firing at all or a huge leak followed by no gas left. Also, it would be noticeable immediately on opening the valve. Last edited by CJOttawa; 05-13-2011 at 07:47 AM.

05-13-2011, 07:49 AM	#6 (permalink)
PB Weasel Sneaky Little Bastid #3 Join Date: Apr 2010 Location: Burlington, Massachusetts	What you said is very true CJ, however my PT only snapped one of the three supports, causing it to hang up and partially vent a couple times per 12ie. Of course when they fail as yours did ( snapping completely in two) the issue is much more noticeable. Good luck with righting your dux short. __________________ My Feedback:http://www.mcarterbrown.com/forums/f...ml#post1449446

05-13-2011, 09:04 AM	#7 (permalink)
WickedKlown2 Space Monkey 615 !!! Join Date: Jun 2008 Location: Middle Tennessee	Weasel,,, I talked to Sergey at LAPCO and he told me the difference in the LAPCO #6 and #6P and the shoulder on the 6P is stronger than the 6... The 6P also uses a undercut cupseal that seals better than the flat face cupseal for the 6... All I use is the Lapco 6P in all my Nelson based markers and when I had my DuckSlide I cut down a Lapco 6P and put in it and never had a problem... Ask TheYoda to see it next time you see him... ------- ----- ------- --------- -------- Short if you are coming down for the Mid-Tenn Outlaw game on the 21st of May I can cut down a Lapco #6P and Lapco Delrin Cupseal for you... WK2 __________________ Proudly Sponsored by Tippmann, FullClipUSA, Hammerhead, CPX Sports and RANGER UP <--Click the Ranger Up link and receive 20% off your purchase MCB - Feedback for WickedKlown2,,, Automags.Org Online Forums - Feedback for WickedKlown2 Last edited by WickedKlown2; 05-13-2011 at 09:14 AM.

05-13-2011, 12:45 PM	#9 (permalink)
PB Weasel Sneaky Little Bastid #3 Join Date: Apr 2010 Location: Burlington, Massachusetts	Well now WC2, thanks for the info. I was told something totally different, I shall promptly smack them in the mouth learn something new everyday __________________ My Feedback:http://www.mcarterbrown.com/forums/f...ml#post1449446

05-13-2011, 03:08 PM	#10 (permalink)
WickedKlown2 Space Monkey 615 !!! Join Date: Jun 2008 Location: Middle Tennessee	Weasel,,, Anytime Bro The reason I found out is there was a discussion one time about the difference so I asked the expert at Lapco to get the answers I was needing... Knowing is Half the Battle WK2 __________________ Proudly Sponsored by Tippmann, FullClipUSA, Hammerhead, CPX Sports and RANGER UP <--Click the Ranger Up link and receive 20% off your purchase MCB - Feedback for WickedKlown2,,, Automags.Org Online Forums - Feedback for WickedKlown2

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search