Cyma AK misfeeding/spiking FPS

[skarra333]

Hi guys,

 

My first time trying to fix an issue myself so all help appreciated.

 

So last game my AK began misfiring, having wild accuracy, then having a very low FPS (firing about 10 meters max), then ceased firing all together.

The site technician told me it was just a fouled barrel, and that there was probably an issue with the hop forcing the blockage.

He cleared it, and told me to get a new hop.

In taking it apart, the hop seems fine to me. I'm really not sure what a knackered one is supposed to look like but it doesn't look noticeably faulty to me?

 

So I put it back together just to see what happened, and now it frequently misfeeds still - every 5th shot or so. The first shots will be ok, then a bb or two will just roll out of the barrel.

Of the shots that actually shoot, I am getting 340fps on most, but every 10th shot or so, it will spike to over 500 FPS - which is of course concerning.

 

I've no idea what the issue is.

When the barrel and hop were off, I did notice on full auto, the air nozzle didn't return all the way to its original position after firing. I'm not sure if that's a factor?

 

It's a Cyma CM0.28, with a plastic TM hop unit, Madbull Python V2 tight barrel. All other internal parts stock running an 8.4v ni-mh battery.

 

Any thoughts and advice welcome.

[ImTriggerHappy]

It sounds more like the hop bucking than the unit itself.

I would guess that feed/sealing lip is distorted.

[Sitting Duck]

When the barrel and hop were off, I did notice on full auto, the air nozzle didn't return all the way to its original position after firing. I'm not sure if that's a factor?

 

 

nope - perfectly normal on semi, by the time guns stops to rest the nozzle has already started to retract again

 

The 500fps reading has got me confused the most though - if you can find how you can squeeze out another 150fps out of the gun I'd love to know

(even if you had 2 bb's coming out chrono would still show 340 or less actually as you firing a bb at double the weight)

 

misfeeding - hop & barrel, place bb in hop and moderate force with pen/thin screwdriver/cleaning rod end should easily pop the bb through bucking lips

as Trigger says bucking lips could be bent/distorted, worth trying a new hop/bucking first coz misfeed problem still exists

so try/experiment first.....

 

if still no joy it may need further investigation inside the box in case broken/loose/damaged nozzle/tappet plate

(I would first check the nozzle/tappet by firing it in auto to get nozzle fully forward right on the point of firing - test nozzle pushing it back/forth for resistance and smooth straight operation etc...)

 

but 500fps

that is a weird one coz wrecking my brain how you could get that increase all of a sudden unless it is shooting 500+ but has piss poor seal most of time

[ImTriggerHappy]

Didnt read the 500 fps bit, cant see how it can spike that high.

[Sitting Duck]

Think there is a Co2 bulb jammed in there or something

it is beyond normal comprehension to me how that is taking place

 

OK it is 455mm barrel so it is gonna get higher fps on "normal springs"

but imported out of box hitting 400fps - maybe 450/460fps with tbb perhaps

(though the madbull tbb and iffy or so-so bb's could be attributing to misfeed/jams)

 

but the creep - heck that ain't not creep that is f*cking joule LEAP ffs

something ain't adding up at all

 

you don't need a tech, you need a priest coz gun is f*cking possessed to me

f*cking weird how you get that jump unless you got high quality JBBG mix of .25's with 0.12's

or something daft like that - think we should run a competition - answers on a postcard to:

 

WTF IS UP WITH THIS GUN Competetion.....

 

Actually - first thing I would do is put back the stock barrel, the cyma barrels are usually pretty damn good

my 28a's which I haven't used but when tested @ 380fps they were bloody accurate indeed

and the cm515 cheapy - barrel is also damn good and the crowning was pretty sweet at first glance

(found a 380mm barrel gave a little more fps than a so called 409mm tbb)

Samurai has said they are good barrels and can confirm two things:

 

1 - he is right on cyma barrels

 

2 - he knows his stuff

 

So I'd consider using the stock barrel once again as not being funny madbull are hit/miss or can be picky for some

(I didn't have sweet joy when I tested one so ripped it out and went with another barrel instead = fed fine on same hop/bucking)

 

Yeah, think I'd be inclined to get the old cyma barrel, a cyma metal hop if you like, new bucking and try that before ripping box open

normally the cyma's are done very well inside so if it ain't broke don't try to fix it

try out the other combo - hopefully she feeds better and more consistent before delving any deeper is my main suggestion

though I am not experienced on AK's coz not used them so not the guru on these bastids at all

[Samurai]

It's 95% the hopup bucking causing this kind of misfeed.

 

The 500 fps is bad. There is no way to up the fps that high with a spring that is not powerful enough to output 500 fps. So the spring is extremely strong in it but something is not working in the gun so most of the time it's only firing 340. I met a gun once where the technician "fixed" the issue with an insane spring instead of finding out why the gun was shooting weak.

[Sitting Duck]

agreed - low fps due to bad seal & chucking in higher spring is a bodge at best

I know shorter mp5k's type guns etc.... need a slightly higher spring compared to longer guns like AK's

but in general you should not use much higher springs to compensate for air leaks/bad sealing etc....

(just making for more work/stress/strain on box/motor than it needs to be)

 

Fortunately I can test a spring in a UAR to get a "rough" idea of what to expect - only a rough guide as creep/drop can still occur depending on barrel

but at least I can get a rough idea of what I might expect so to speak

If I'm dropping way off the mark then I know I got problems I need to check

(love my UAR x x x)

[skarra333]

Hi guys.

Thanks for the advice.

On very close inspection the bucking looks off - not by much mind, so I'm assuming that's what it is.

I just put 150 rounds through it and not once did it go higher than 340fps, but I assure you all that 500+FPS - 2 something joules was coming up last night (element ex236 chrono). I could have imagined it but it seemed very real at the time.

I'll sort the buckling then and see what that does.

I don't have the original barrel though so will try to make do.