M4 Piston sticks half way back in the gearbox after each cycle.

[roryrocker13]

Long story short I have a cheap m4 I picked up for £100 on sale with the intention of upgrading it a little and lending it out as a gun for friends to use. It has been sitting for a long while under my bed and the other day I had some M4 parts that I acquired when upgrading my main gun. I replaced the cylinder for a 5ku type 3, the piston for a ICS POM piston and I put in an unknown brand of air nozle along with an unknown brand of cylinder head that I had lying around I also put in a new motor (I think this could be the issue here unsure though) and away I went. Shot beautifully for something that probably cost less than £150 but what is happening is when I shoot the gun through the air port in the side of the cylinder I can see that the piston is stuck half way back in my gearbox (so the full cycle is piston half way back, pulled back by gears, released and then sticks half way back again). I think it could be a tooth on the gears catching the pick up tooth of the piston but I am unsure. I also really dont want to remove any piston teeth if I can avoid it and I also have very basic tools (no dremel or angle grinder) what do you guys think? Will my spring be getting damaged as a defect of this? What teeth would need removing if that is the problem. I have tried 3 different pistons in the gun and they all do the same thing. Could it be the G&G high speed motor that I found in a parts tray and stuck in my gun or is it just a poor gearbox from a cheap gun?

Edit: The tappet plate is also stuck back at the end of every cycle so the air nozzle does not move (you know what I mean).

[BikerEddd]

Re: M4 Piston sticks half way back in the gearbox after each cycle.

What you are getting is called 'overspin'. It is caused when residual momentum in the system causes a half cycle even after the motor has stopped. In extreme cases it can cause semi auto to complete 2 shots every time (one intended then the second on overspin). It is perfectly healthy and infact is a sign of an efficient and low mechanical resitance setup so give your self a pat on the back

There are ways to get rid of it if you want. Active braking being the main one but unless you are getting so much overspin it is double shotting I wouldnt bother

Sent from my SM-G920F using Tapatalk

[Sitting Duck]

Re: M4 Piston sticks half way back in the gearbox after each cycle.

The new motor - probably a faster neodym is the likely cause of this on a fairly std setup box with m100 spring...
or higher juice - but the box is running faster and with this you get over spin or piston being half pulled back

not really a massive issue if moderate overspin - but when done use a used battery to reduce overspin when storing gun
(or in this possible pre-cocked state the spring's tension will decrease - all springs lose tension but pre-cocked will lose tension quicker)

would guess you are maybe getting about 25rps and at this speed you will get some overspin
at 30rps she will try to double fire - if this occurs ease up coz you are getting close to Pre Engagement

it is ok to use as it is with a slight over-spin - just use a lower battery to park the piston better and not pre-cocked

going 30+ will require either AB or stronger m120+ spring & Short Stroking to bring down the higher fps to UK spec
(tbh keep gun in 20's perhaps mid 20's is what many just go with as easier and less stresful for box and owner in upgrading)

The tappet/nozzle retraction is normal - in fact the nozzle starts to retract even before the sector gear engages the first tooth/pick up
you are not going to prevent this unless you use AB mosfet or such a slow setup/battery pushing 10rps
(yeah I think you will just accept some nozzle retraction is quite normal like the rest of us)
So don't worry about nozzle retraction - omg my gun is massively over spinning - ergh no even if piston just kissed the initial pick up tooth = no piston movement at rest - you will have the nozzle retracted about 90% of its full travel I'd say
If your nozzle is fully forward then that would indicate the piston is close to releasing for a second cycle/double firing as when nozzle fully returns then there is only about 4 teeth left to retract piston before she fires - just enough for nozzle to settle & seal hopefully before piston thunders down cylinder creating a nice pop of air to propel little round balls of hell towards your opponent

Normal toy gun operational crap really

bit of over spin = no worries but don't push it much more that she double fires
nozzle retraction = perfectly normal

[roryrocker13]

Re: M4 Piston sticks half way back in the gearbox after each cycle.

Ok thanks guys.
I think im gonna leave it. It does double fire (I didn't include it in my description) but it only occurs like 1 in 100 shots if even that. I might try some of your tips like using the low battery for when I am going to store the gun, I am gonna wait till I get a couple hundred shots through it at least before I start to decide if I should change the spring out for a more powerful one but for now I think its fine.

[Sitting Duck]

Re: M4 Piston sticks half way back in the gearbox after each cycle.

Well get her chrono'd first and go from there

double firing can happen even on a 7.4v lipo if tweaked with a low spring
eg: ak 47 with 455mm may only need a m90/95 spring to hit 340-350fps
whack in a neodym high speed motor with good wires/mosfet/deans and she will over run at least very easily

Usually though at about 25rps over run starts to appear - can double fire if lower spring etc like ak above
30 rps double cycle can happen - I'd watch for this and try to avoid - so get chrono'd and up spring if needed

end of the day it is probably the "juice" as well you are putting through her....
7.4v often is all you might need in a tweaked gun to hit near 20rps
11.1v will give you the 30's - heck even 40+ on some setups but 30+ like I said you will need more work/higher spring SS etc....
9.6v nimah's can be a nice stop-gap between the 2 lipo options
and then more stuff into the mix is the burst or C rating on lipo's can make a difference too
a 20c might be ok but 25c, 30c, 35c battery can push up the rps a bit just by increasing the burst

So get her chrono'd, see what she shoots like and use a battery that suits her without the double firing if possible

Summing up - getting a gun to run faster & faster is not quite as simple as pushing faster motor or more juice through her
for starters if AoE isn't corrected at least - then be ready for a stock gun's piston to strip or most likely just snap off at pick up tooth
(done it "once or twice" taking liberties myself)

just go easy tbh - 20's is easy and don't need much work at all and lasts well
30's - well that needs a bit more care to work properly and last