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Hop-Up Destruction and Malformation


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#1 Jatodude

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Posted 02 May 2013 - 04:32 PM

Recently I removed the LM4 barrel, and disassembled the hop-up unit to diagnose some issues. It became very clear when it was disassembled down to the bucking cover that there's something really wrong. Rather than return to the off position the hop up remained protruding into the barrel. In an effort to adjust the hop-up by pressing down where the tensioner should be, it simply protrudes into the barrel 100% with a rapid pop, or remains completely off. It does not take well to pressure being gradually applied. I previously thought the fins were torn slightly, but the two nubs actually where simply resting inside the barrel at an angle of approximately 20 degrees. I noticed a slight tilt of the fins from factory, very small but just barely noticeable to the eye. Also I found that the bucking has a substantial chunk missing out of the bottom lip near the feed ramp. The bucking also does not firmly rest on the barrel, it seems like it has swollen and become loose.

What I need to know is how to prevent something like this from happening with a new bucking. Perhaps it got too much lube and became malformed and swollen? And what could could have caused chunks to get torn off of the bucking? I can upload pictures to better convey what I'm saying, if necessary.

The compression still drops and does bb rolls, like it would with a worn out stock piston o-ring. I think the bucking is the culprit. I've replaced the o-ring to the Harbor Freight one as Boobie suggested, and lubing everything generously helpd the cause slightly, but not for more than a magazine. Is there any trouble shooting that can be done other than just buying a new bucking? I wouldn't want the same thing to happen again, or fail to find the elusive solution to this compression loss problem.

Edited by Jatodude, 02 May 2013 - 04:34 PM.


#2 -wigs-

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Posted 02 May 2013 - 06:12 PM

Is your inner barrel pushed in further than it souls be? Say from an after market flash hider that doesnt fit around the inner?

#3 Jatodude

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Posted 03 May 2013 - 05:58 PM

I don't think the flash hider damaged anything. Looking at it, nothing protrudes into end of the outer barrel. It's just a regular bird cage flash hider.

#4 akiraspeedstar

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Posted 03 May 2013 - 06:45 PM

I don't ever recommend running silicone with your propane due to the risk of getting it onto your bucking. If you must run silicone with your propane then run very little. If you're running green gas then it's already in there and it isn't much anyways.

Also what are you using to lubricate your bolt? and how much? I had a friend who recently purchased a new LM4 and he had jamming issues and all that. When I opened his gun it was excessively lubricated (unsure if he did it or from the factory) and that silicone had gotten onto his bucking. The bucking was deformed and swelled and once he replaced the bucking the issue was gone.

Also after market barrel or OEM? I had a dud barrel from RAT where the hopup opening was too small. I had to take a file to it so that I could open it up.

Did you have any jams? If so that's probably the reason that you see the bucking damaged.

#5 Jatodude

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Posted 04 May 2013 - 03:48 PM

I'm thinking silicon had something to do with it. The bucking looks very swollen, perhaps from when I was dealing with the o-ring issue. I used the silicon that came with the AI propane adapter for some time, then switched to 30wt RC shock oil. I had one or two jams in the few thousand rounds I've shot. But what worries me is that from very early on, say around 500 shots the bucking was already tilted. Eventually it swelled so much that the hop up tensioner never even touched the bucking. Can a bucking become swollen that fast?

Another random symptom, on full auto the bb drop issue would start happening and occasionally the bolt would like, hang up or something and blow all the gas out the ejection port. I figured that this was from the piston prematurely disengaging from the hop-up unit and getting pulled back with the bolt.




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