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Fps Issue


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#1
Glavier

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Hey, guys.  I used to work on my own guns back in the day, and I know I'm a bit rusty, but I'm having an issue where I'm left scratching my head.

 

The rifle:

 

MP5.  I wish I could say this were all one brand, but this was one I purchased used years ago (and prompted a bad sellers review).  The body is ICS.  The gearbox is an older JG Box, gears, etc.  I've had to replace the tappet plate, the wiring harness, the trigger, and other misc parts.

 

I'm currently having an issue where it doesn't matter what spring I put in (110, 120), I cannot seem to exceed 310 fps max.  I did upgrade to a Hurricane NB cylinder kit.  The cylinder/piston combo had great compression (tested by hand).  The only thing that I can visually see is that the cylinder is maybe 1/16" shorter than the space in the gearbox.  I've visually inspected the GB and do not see any cracks or gaps elsewhere (especially when firing).  My next step is to replace the gearbox, but I wanted to come here first and see if you guys could provide any insight.

 

TIA.


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#2
Hanback

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Sounds like a compression issue between the gearbox and hop-up. First things first, inspect the air nozzle for damage. If not damaged, ensure it is the proper length nozzle. Too short of one will cause MASSIVE compression and even feed issues. Replace if need be with the proper length nozzle.

 

Are you at 310 consistently? Chances are since the gun has sat for a long time that the hop-up bucking is trash. This can cause numerous issues itself. Probably should replace the bucking regardless. Upon reassembly of the unit, make sure everything goes together properly and in the right positioning.

 

Lastly, on reassembly of everything, check the mating of the hop-up to the gearbox. Ensure that the alignment of the two is correct and that the hop-up is sitting back into the gearbox as it should.

 

Next up: What cylinder type did you install and what is the barrel length? This plays a big role as well. A standard cylinder to barrel volume ratio to stick around is 2:1. Trying to run a type 2 cylinder on a 400mm barrel (using an exaggerated example) will be lucky to generate an fps of 325 even with an m150 spring. 


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#3
Glavier

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I believe the barrel is the standard 229mm length, and it's a 6.04.

 

Here is the kit I used for the cylinder: https://www.airsofts...linder-set-mp5/

 

Replacing the buckings on all of my rifles is next on my list as well.

 

As far as alignment of the gearbox to the hop up unit, everything checks out visually, but when I block the barrel, i'm not feeling a very strong blast of air (compared to just blocking the air nozzle directly.


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#4
ghostface320

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Duuuude Same problem! I just put in a new spring two days ago to my mp5 and its shooting 310 when it should be 380! The F%#$ right lol?


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#5
ghostface320

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I'll have to check to make sure the hop up is sitting properly on the gearbox, but its feeding fine so idk what else my problem could be.


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#6
ghostface320

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come to think of it my buddy put in a new air nozzle and I bet thats my problem...


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#7
Hanback

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Ok, cylinder and barrel look to be matched, should be good there. The best bet is going to lie with the air nozzle, since you said that two different rated springs yielded similar fps. The hop-up bucking may still be part of it (very likely), but similar fps output on different springs is indicative of a compression loss somewhere forward of the piston/cylinder and behind the hop-up bucking.

 

I would disassemble and redo compression testing of the cylinder and piston with air nozzle on the cylinder head. This will either show compression loss between the nozzle and cylinder head or rule this out. There will likely be slight compression leak here, but so long as it isn't significant, it will be ok. If it checks out, then the issue is likely between the nozzle and hop-up. Nozzle to hop-up compression loss is a bear. Best bet is to try a longer nozzle. And longer is defined at 0.2-1mm longer (calipers help here). Don't go too long though or feeding will be wrecked.

 

Before trying a different nozzle though, fully disassemble the complete hop-up unit and then reassemble ensuring everything is placed perfectly correct. Something, like the bucking, could be preventing the nozzle from traveling fully forward to seat properly to create the necessary air seal.


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#8
ghostface320

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So I think my issue is with the cylinder, I noticed with the new spring, since it is stronger, when I shoot it knocks the cylinder somewhat loose and Im loosing compression, that and the O-rings prob need some work.


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#9
Glavier

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I appreciate the input - this kit did come with a new air nozzle, so I'll see if I can find the older one in my bag o' parts.  I'll keep you posted as to my findings.


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#10
ghostface320

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Yeah, please let us know what you come up with, it might shed some insight into my issue as well.


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#11
Glavier

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Okay, with the nozzle swapped out, it gained me another 10-15 fps.  I know the buckings need to be replaced, and that's my next step, but it gives me something to work with tomorrow for my +1.

 

Thanks again!


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#12
PELI

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ICS is good at throwing in proprietary parts like tappet plates, air nozzles , selector plates and hopuos :(

 

i have a big box of MP5 parts including 1 or 2 Ics TM and some JG part including hopups if you want to try one out


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#13
Glavier

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Cool, I appreciate it. I'm debating about coming back down to Bloomington next Saturday for some B/S/T action.
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#14
ghostface320

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So weird with my issue, I swapped out the upgrade parts and put the old parts back in (piston, spring) and the problem fixed itself.  The FPS started going way back up to where it was supposed to be, so maybe i wont try and fix things that aren't broke anymore lol.


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#15
Glavier

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I tried going with the whole, " if it ain't broke, don't fix it" angle in the past. It gets boring after a while.
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#16
ghostface320

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Very true lol


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