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Screw Loose

CYMA CM 506 FPS issue

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Morning all! 

 

Picked up a Cyma cm 406 and took it to my first game last night, when cronographed it was only shooting at 240 FPS! Advertised power is nearer the 400 mark, I expected it to be a bit off from what they claim however site limit is 340 and ideally would like to get closer to that. Is it just  a case of a new spring? Stripped the gun back last night and appears it may have been cut, or is there anything else I can check/do to increase the FPS to site limit? 

 

Thanks in advance 

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Ouch.  Yes, mine came clipped from PatrolBase, but it's shooting ~330.

 

Did yours come from a site with "BB" in its name?

 

Sitting Duck is the chap to ask, but I believe they take generic AEG springs, and an M100 - this, for example - should see you right unless there are air leaks or other factors in play.

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Thanks for the quick reply! No I bought it second hand (probably even worse). I've ordered a generic spring to see if I can make it fit for the sake of a tenner, probably won't be long until I upgrade though, quite fancy the full metal versions however I  might hold out for a bargain! 

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Ah.  I guess it could be anything then.  Have you checked the air seal on the piston O-ring, cylinder head and nozzle?

 

No broken or missing teeth on the piston?

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Check all the seals - the piston o-ring in most stock guns are crap

the cylinder head is quite loose - ptfe tape & trim off excess after refitting

(these won't boost fps by loads & loads but as Tesco says "every little helps")

 

The red SHS M4 nozzle is a tad loose on cyma tappet - try the plastic ZT100 SHS nozzle

(if you consider upgrading the std clear nozzle)

 

Check for compression - basic common sense stuff

 

Fit the M100/M105 spring - if one end has tighter coils these should go at back of gearbox

(spring size varies and a longer spring doesn't always guarantee a stronger spring

 

Really daft stuff but make sure the cylinder port is fitted correct way round - the hole at the rear

(people have replaced the cylinder back incorrectly and wondered WTF ????)

 

Slap some grease in there, check it all looks & seems to operate ok blah blah blah....

(could go on with tweaks/upgrades but will leave it there for now)

 

NOW - if the box & compression is all good then HOPEFULLY your fps should be up to 330/340 -ish

BUT.......

You may also be losing fps through the hop/bucking......

If the hop is not pressed back against the gearbox then it won't seal 100% or pi$$ air from nozzle to bucking

so ensure the hop unit is fitted up fully against the gearbox & perhaps a hop spring/o-ring(s) to ensure it stays sealed

If the bucking is torn then you will lose fps also - yup just coz spring/compression is all good in the box - you can still lose fps

 

Hopefully you can fix leak and get the fps back up to what it should be with a bit of care & attention 

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16 hours ago, Samurai said:

If it's missing that much fps, then high chances are that it's not the spring.

Before you spend a fortune on trying to figure out what it can be, give it to an AS mechanic to check it.

 

Ninja'd again - sounds like a hop/bucking seal issue to drop 100 or so

but being s/hand who knows what has taken place prior to this....

 

should hit near 395fps on the m120 which is too hot but can be used if ya want

(drop the metal spring spacer in piston & SS 2 or 3 teeth and you can still use the stock spring as is)

((EDIT - can only Short Stroke if you are replacing piston with more/full metal teeth rack - forgot that bit))

(((but have downgraded a 400fps stock gun this way keeping original spring as is with new piston)))

 

Sounds like a hop leak - might not be reassembled against the gearbox or somebody had a go at something badly

maybe a torn bucking, tried to tweak it and torn it or fitted badly or somebody hacked the spring loads too much

 

I mean there are tests you can do to narrow some symptoms down a bit but hard to run through in text

but basic common sense elimination stuff

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By where the seal to the hop unit is is that literally the clear plastic piece from the gearbox to the barrel (via the hop)? If so I've out the two together and there's a rather large size difference, could this be the problem? 

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THIS.....

 

Related image

 

if the hop doesn't sit right up against the gearbox then air will leak

 

when a mag is inserted the mag might try to pull the hop away from box slightly

sometimes there is a spring on hop unit to push it back to keep the seal or o-rings to ensure hop remains sealed....

 

Image result for m4 hop unit fit

 

see the small spring on black hop unit or larger spring on clear hop around barrel

or o-rings does the same job...

 

Image result for m4 hop unit o-ring mod

 

NOW - this all well & good but if bucking is fitted badly or torn that will pi$$ out air also

then if nozzle is not sealing against bucking lips - but unlikely if all still ok & stock parts used

(usually you can get problems if you use a shorter nozzle or change hop unit to a prowin & not so compatible etc...)

 

but who knows as gun is s/hand and f*ck knows what the previous owner did/didn't do

hence if stuff is still crap then like Samurai suggested seek out more help from an experienced tech

They should be able to quickly identify what is ok and what is not operating correctly

 

Hop units and compatibility issues can really make a big difference

some work for some guns and crap for others

the nozzle is a std 21.4mm length in the Cyma but may/may not seal great with all hop units

if anybody messed with bucking or hop that is where you can lose 100fps easily

 

Most likely the hop unit may not be sealing against the gearbox 101%

well I'd pay close attention to that too when reassembling the gun

no need to go nutz stripping more stuff but ensure the hop is sitting right back against box

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I've got no spring or anything like what you've shown me there - could that be the cause then? Where would I get a spare? 

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2 minutes ago, Screw Loose said:

I've got no spring or anything like what you've shown me there - could that be the cause then? Where would I get a spare? 

 

out of a clicky bic ball point pen or something ???

 

it isn't a bespoke special order custom made spring from Germany or anything

there are other ways/methods

 

I'm just giving you examples of stuff/problems and what you can do to help solve issues

BUT most of all this is YOU paying very close attention to how all the crap fits together & operates

Teching isn't really about buying the expensive parts so much but more how the stuff actually operates

like cheapo ptfe tape, washer to correct AoE, bit of grease, the rest is time checking out stuff works properly

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Update - resolved it! After much faffing about with springs and seals and everything, gave up and put a new bucking on. Fixed straight away and now shoots somewhat consistently at 338fps! Thanks for all the help everyone! 

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