Diagnose this sound - M4 aeg

Discussion in 'Gun Building, Modifications & Repairs' started by babachimbu, Mar 23, 2017.

  1. babachimbu

    babachimbu New Member

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    Santa Fe
    My King Arms PDW M4 variant quit firing and makes a sound like someone playing a washboard, or the sound of a ratchet, rather than the normal crisp firing sound. You can hear it here: https://youtu.be/JuPuffuPPEI

    Any idea what the problem might be?

    Cheers!
     
  2. Lefse

    Lefse Well-Known Member Supporting Member

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    The nozzle moves, so all the gears cycle normally, but the piston is not pulled back, ergo the piston is stripped.
     

  3. Sitting_Duck

    Sitting_Duck Well-Known Member

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    shredded piston

    the sector gear is turning - hence the nozzle moving
    so all gears are turning of a sort & pulling tappet plate/nozzle back n forth...

    BUT - piston doesn't seem to move in rear window

    Likely shredded piston from PreEngagement
    Or
    Pick up tooth/back of piston snapped off

    been pushing the gun much ?
    sounds like you will need a new piston in there at least
     
  4. Sitting_Duck

    Sitting_Duck Well-Known Member

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    Ninja'd by Lefse :D
    wtf - 20 characters - my posts are way longer usually
     
  5. babachimbu

    babachimbu New Member

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    Santa Fe
    Have sometimes used 11.1 lipo, though not recently. I've had it more than a year, but haven't used the gun much generally. Thanks for the info.

    I haven't opened a gearbox before so a new adventure awaits.
     
  6. babachimbu

    babachimbu New Member

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    Santa Fe
    Thanks for the quick responses. Any suggestions for replacement pistons?
     
  7. LucidAirsoft

    LucidAirsoft Member

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    Canton
    Anything with a full teeth tooth rack
     
  8. Lefse

    Lefse Well-Known Member Supporting Member

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    An SHS or CYMA steel rack piston should last forever in your setup.
     
  9. Sitting_Duck

    Sitting_Duck Well-Known Member

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    Also perhaps read up about correcting Angle of Engagement
    (shifts the piston back slightly so teeth engage better is rough explanation)
    Think a bit of juice, bad AoE & weak piston played their part in this mishap

    Depending on the gun it should be ok to run 11.1v if you haven't got a fast motor
    If you have a very fast high speed motor on 11.1v you run risk of Pre Engagement
    RPS
    If a gun stays in the 20's or mid 20's you are ok
    mid/high 25-30's then it can run on or overspin/cycle
    A little overspin is ok but if she fires twice on semi often then you are starting to push it and risk of gears hitting piston's teeth before it has returned = PE

    I doubt if you are hitting this on a std motor or a slightly torquey motor on 11.1v
    But might be near it if using a very quick motor
    Yes you can run guns this fast - even faster but a bit more planning/work required
    There is a limit to push mostly stock guns - if she overspins a lot or fires twice on semi, then consider that the gun is almost talking to you saying ease up a bit

    Reason why I'm mentioning this, is that if you fit a full steel rack and you was/still push the gun a bit too quickly near PE then you can smash more than just a piston as all gears are metal and can do some more damage than just a piston
    Yes a metal rack piston is way more durable and tbh I doubt if you was going that fast but just pointing this out in case you have a light spring and very fast motor on 11.1v

    Most likely weak piston & bad AoE that caused it to go on 11.1v than PE
     
  10. babachimbu

    babachimbu New Member

    19
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    Santa Fe
    Haven't seen double shots on semi, but I sure appreciate the thoughts. Will read up on angle of engagement.