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free move of hop up unit


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#1 'Toaster' To

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Posted 15 August 2013 - 01:32 AM

Hi guys,

I've just replaced the hop up rubber on my KWA KM4S tactical and noticed that unlike other AEGs the hop up/ inner barrel assembly seems to meet some resistance and does not spring back on the spring. It seems to be getting stuck somewhere but from what I can see, there is nothing stopping the hop up unit from moving back. this is just one thing I'm looking into as part of a bigger problem in that the rifle is shooting with the accuracy of a shotgun right now!

Are they always like this? I have a JG M4 System also and the hop up unit on that springs back as it should.

#2 niko_gpsy

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Posted 15 August 2013 - 04:05 AM

Hey Toaster

Welcome to the KWA Forums.
Did you make sure that the barrel locking "C" clip is installed correctly and that it's flush with the outer surface of the hop up unit?

#3 'Toaster' To

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Posted 15 August 2013 - 05:28 AM

Good call Niko. I'll check when I get back home later. I'm also suspicious of the FPS as the spring has very little tension. Can a extremely low FPS cause my symptoms with my hop up?

#4 gcw360

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Posted 15 August 2013 - 08:21 AM

View PostFrom 15 August 2013 - 01:32 AM:

Hi guys,

I've just replaced the hop up rubber on my KWA KM4S tactical and noticed that unlike other AEGs the hop up/ inner barrel assembly seems to meet some resistance and does not spring back on the spring. It seems to be getting stuck somewhere but from what I can see, there is nothing stopping the hop up unit from moving back. this is just one thing I'm looking into as part of a bigger problem in that the rifle is shooting with the accuracy of a shotgun right now!

Are they always like this? I have a JG M4 System also and the hop up unit on that springs back as it should.
While a torn or incorrectly installed/incompatible bucking can result in a drop in FPS, there are a whole host of other things that can do the same thing from a stripped piston to a damaged or wore piston o-ring.  Let's get the barrel returning to it's proper position first and see if that helps and move on from there.

I had the same issue you are having with the barrel not returning to it proper position.  The cause was a tiny bit of "flash" from the casting process found inside the outer barrel.  This rubbed against the inner barrel an stopped it from moving easily (FYI, this back and forth movement is the main reason I do not use, nor do I recommend o-ring to stabilize the inner barrel in a stock KWA set-up.  If the o-ring impedes the travel of the barrel, it can affect FPS.)  If you find this to be an issue with your gun, you can fix it by passing the appropriate sized flat head screw driver down the outer barrel and use it to scape off the extra material by turning the screw driver.  Let use know if either Niko's or my suggestion works for you.

#5 'Toaster' To

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Posted 16 August 2013 - 01:09 AM

Ok Guys I've had a good look at it last night. it appears your were right about the C-clip Niko. It wasn't sitting flush and taking that off the unit and sliding the assembly back in has help and it bounces back. It does however 'stick' when it's push all the way in.....

gcw - There were no burs inside the outer barrel so that's clear. i don't have a chrono so I setup a short 2 Metre range and used a microphone to measure the sound from when the BB leaves the AEG to when it impacted a hard surface and then calculated the fps which turned out to be 35-40 fps!

I'm going to change the spring to one which I know is around 390 fps because this spring is so floppy it's like a slinky!

#6 'Toaster' To

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Posted 21 August 2013 - 02:36 AM

Ok so I;ve changed the spring and it hasn't really helped with the FPS. I suspected an air leak so I've checked there was no air leaking out the BB feeder when firing. I had to change the piston head and cylinder head which has improved things but the new cylinder head seems a bit on the loose fitting side. My next port of call will be to check the cylinder head for signs of the air leaks and the air nozzle.

*SIGH*




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