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Logan Nagel

Compression problem in m4 build

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So, I'm building my bro a custom m4, and everything appeared to work fine, until I shot it and it shot at a very low fps, very low accuracy, and occasionally shooting double. Same in full and semi mode. I looked, and it looked like there was some wear on the top lip of the air nozzle, which leads me to believe that the air nozzle could be hitting the top of the hop up lip? If that's even the problem, I have no idea on how to fix it, I've never encountered something like this before. The upper and the receiver are from different guns, could that be causing a compatibility issue? Help!!!

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There could be an alignment issue between the two different receivers.

 

One options is to try and shim the back of the gearbox just underneath where the buffer tube attaches. Use something like small squares cut from a cereal box. Stack multiple if needed. For the most part, the gearbox is pretty well fixed in place by the receiver pins, however sometimes just a little pressure in the back is enough to tip the gearbox down in the front to align the nozzle properly. I have done this on one of my guns, and while it seem like a hack job, the gun has functioned fine.

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Alright, that sounds like a good idea. Thanks for the advice!

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*Sigh*, didn't work. I stacked enough cardboard to make it pretty hard to get the gun together, but it didn't make any difference. The air nozzle probably just needed a minor tweak. So, back to square one.:( It's still getting very low fps, hopup doesn't appear to be doing anything, but I've changed out the hopup/barrel assembly several times and it does the same thing. Its shooting at an insanely high rps now that I put a 40K rpm motor in it, but still shoots like crap. 25 feet or so.

 

Hmm, I cheated and got the gearbox pre-upgraded and assembled. Think I should check for a problem inside? Idk what it could be, maybe the piston or piston head.

 

Edited by airborne101

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Hmm, I cheated and got the gearbox pre-upgraded and assembled. Think I should check for a problem inside? Idk what it could be, maybe the piston or piston head.

 

Pre-upgraded & assembled where? If from a store, that doesn't mean much in terms of quality, and you can certainly have gotten a lemon/dud.

 

http://www.airsoftforum.com/board/topic/220266-miscellaneous-internal-tweaks-guide/

Follow the link to the compression test and do that to see if you can identify where the leak is if you have one.

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Right I had the same issue with a custom built M16 I did... rubbish range, rubbish fps etc etc....

 

After taking apart the gearbox a billion times checking cylinder head o'rings and piston o'rings checking compression over and over...

 

Went from standard hop units to Lonex ones to prowin hop units... many different barrels from madbull to pdi's....

 

Many different rubbers standard black, G&G green, madbull red to prommy purple....

 

Turned out to be the magazine well.... my magazine wasn't meshing up with my hop unit properly a few well placed bits of velcro in the magwell and hey presto!!

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So, I did Star_folder's compression test with the paper over the feeding tube, and sure enough, the paper shot up like a rocket!:D Now I just have to figure out how to fix it... So, the problem is either the hopup to air nozzle seal or the bucking to nozzle seal. I hope I can have this fixed in time for Operation Plague here in Wisconsin.

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Try sanding the tappet plate, it's in the misc tweaks thread.

 

Also if you have any spare parts to switch out, you can try exchanging hop up units, air nozzles, buckings, etc and see if you can isolate the problem.

Edited by SlabGizor117

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Good idea, didn't think about sanding the tappet plate. I'll check in this order; bucking, air nozzle, tappet plate. Hopefully I can get that all done tonight, along with my friend's G&G m4.

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Try sanding the tappet plate, it's in the misc tweaks thread.

 

Also if you have any spare parts to switch out, you can try exchanging hop up units, air nozzles, buckings, etc and see if you can isolate the problem.

 

What would this do? It's not your tappet plate.

 

Your main issue is you mixed and matched two different receivers. Airsoft items are not Universally interchangeable.

 

Is the spring still on your hop-up? If not put some rubber bands on the front of your hop-up to help it push back against the mechbox.

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I don't think it's the parts from two different m4s, because now I'm having the same problem in another custom dmr m4 I got in a CPO mystery box, just not quite as bad. Leads me to believe maybe the techs didnt want to deal with it, and, that the problem might not lie in the mismatched receiver and upper, but something internal. In the first gun I put in a better air nozzle, cylinder, and piston, nothing. In both of them I tried a couple crappy hop-ups and buckings I had laying around, still nothing. If nothing else I think I'll buy another couple buckings and hop-ups to try, because right now I'm just getting really pissed of by this stuff. It's definitely a compression problem, because the gun failed that paper test before.

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I think I've isolated the problem to a air nozzle to bucking seal problem. A very bad one. What's the best way to fix this? Teflon tape? Harder bucking?

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Ok, sorry, I've never actually heard the term packing before. So does anybody know how to fix a bucking to air nozzle leak?

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NOBODY'S EVER HEARD THE TERM PACKING BECAUSE IT'S CALLED A BUCKING

 

-rant over-

Several companies, including Prometheus, PDI, and Systema (which are older companys) call them packings. The term is out there and has been used. Get over yourself.

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So that's a no then? Lets not let this topic go off on a tangent. Is this problem repairable, or am I screwed?

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NOBODY'S EVER HEARD THE TERM PACKING BECAUSE IT'S CALLED A BUCKING

 

-rant over-

 

Slab...gonna have to pound you for your attitude...you are a newb! Don't pretend you know anything about Airsoft, be nice, be gentle and be correct.

 

Bucking is a "made up" term from a translation error that occurred back in 1997 by SystemA.

 

While packing is a proper term meaning a "rubber" seal around an "open pipe".

 

They corrected the term:

 

sys_hop.JPG

but the people that did not know better continued the improper terminology and it stuck (because many were ignorant). Some company's continue to improperly spell it sometimes. Example below.

 

airsealcamberpacking_main.jpg

But, most spell it correctly

 

pm-hop-scp-aeg_3_mark.jpg

 

main.jpg

 

Notice they are all Asian (Japan mostly).

 

Anything else like US and low end makers still use the "stuck" name of "bucking" (gee, that says something about Airsoft items in the US).

 

And and a side note...the French call it BAXS...they think they invented the technology...which they didn't.

 

 

 

 

Edited by Guges Mk3
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I'm a little apprehensive about grinding the tappet plate, but if that's my only option, I'll go for it. Hopefully I won't :censored2: it up.

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I sand the whole front of my tappet plate all the way across. I place some 120 grit emery clothe on a flat surface and slowly run the front of the tappet plate across it checking every so often. Dial verniers help out a lot here to check the width before starting. I usually take off about .025"-.030" from the front. locprecision.

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*Update*- Sanded the tappet plate; didn't work... I'm going to have to try the old reciever. Shoulda done that in the first place, but the new one had my punisher skull on it. >:(

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*Update*- Sanded the tappet plate; didn't work... I'm going to have to try the old reciever. Shoulda done that in the first place, but the new one had my punisher skull on it. >:(

 

Yeah, I knew that wasn't going to help. Is this still the Javelin upper you are messing with?

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