Semi-auto over-spinning and occasionally burst fires

[Rogerborg]

CYMA CM.516.  Poor thing, I should have left it well alone.

It was functioning fine until I threw in an 11.1V 2200mAh 25/50 lipo, then it overspun and double-fired a fair bit, although I only tried it briefly.  Changing down to 7.4V or trying the stock 8.4V Nimh though I'm still getting some over-cycling, and occasionally it fires a full auto burst while set on semi auto.  I did have the gearbox apart to check the shimming (which was fine from the factory) but it was functioning fine afterwards for thousands of rounds: I only saw this issue after putting in that egregiously meaty lipo.

The fire selector switch doesn't seem to be creeping past semi, and I've held it firmly in place while triggering, but am still getting the issue.

Things I have tried:

Watching this.

1) Checking that the cam on the sector gear and the semi auto disengagement lever aren't worn.  They don't appear to be, and the gun has probably only had 8000 rounds or so through it, but I have no grounds for comparison.

2) Prising the trigger contacts very slightly apart.  I don't want to go wild with this as it seems like it would be likely to cause arcing or misfiring.

3) Checking that nothing is broken or missing - I can't see anything obvious, all the springs and screws appear to be in place.

One thing I've noticed is that when the semi auto disengagement lever operates, the pressure from the little return spring can push the selector plate slightly further along towards full auto.  I can see this with the gearbox out of the receiver, it's a very small movement, and I'd hope the selector lever on the outside of the receiver should prevent this when it's all back together, but I can't be sure since I can't observe that directly.

In unrelated news, while I was putting it all back together I managed to drop the whole gearbox and crack the end off my (plastic) air nozzle, so the box is currently still out of the gun awaiting a replacement nozzle.

Is there anything else that I can try if it's still misbehaving when it goes back together?  I realise that we're talking moving parts with tolerances of fractions of a millimetre (how do these things ever work?) and that I'm going to have to do a bit of trial-and-error to narrow it down, but any suggestions would be gratefully received.

 

[Adolf Hamster]

is it just free-wheeling after the cutoff or are we talking proper full auto?

my m4 has a habit of double firing when on a fresh 11.1 due to free-wheeling after the cutoff (turns out amazon's idea of a "braking" mosfet isn't the same as my idea of a braking mosfet)

 

[Rogerborg]

Bit of both. Fairly regularly doing a partial 2nd cycle, and occasionally going full retard auto.  Typing it out loud, it's got to be the cutoff lever, right?  I'll have a closer look at it, but it doesn't appear to be damaged or missing anythhing.

Anyway, the nozzle arrived today, great next-day service from https://www.ak2m4.co.uk/ (I should probably have bought another cutoff lever while I was at it...) so I'll throw it together and see if it's still going mental.

 

[Sitting Duck]

Yeah - likely worn Cut Off Lever.....

spent 20 mins searching for pic of old/new v2 COL

but coz photobucket is w@nk the picture is gone

it was a good pic somebody took showing the difference in worn/new

the old COL still looks sound - until you put it against a new one 

The problem is stock COL in many cheapo guns are made from DairyLea alloy

This just wears out asap as if you was running a dremmel against it

Stock COL's are often non ferrous/non magnetic and file away or wear easily in COL

Ideally you want to check COL is ferrous/magnetic to reduce hassle further down the line imho

$hit happens and stuff still wears - but alloy type COL's will wear out sooner if you shoot loads on semi

The way the COL works like the old fulcrum/lever bollox.....

Not doing the maths crap but you have say 0.5mm wear on sector side then trolley loses 1mm or more lift

Losing just a smidge of COL will cause poor or late disengagement of trigger trolley etc.....

You have worked this out already, but the point is just a moderate bit of wear f*cks it all up

A soft alloy COL wears quickly on semi and for the life of me I can't understand why they bother

I've recently come to the conclusion that if you intend to use the gun a lot especially on semi

Then replacing the COL when teching/upgrading is wise/worthy consideration

(you have the bastid in bits anyway so it is false economy imho)

At the very least we should check if the COL is magnetic and if not then drop in a SHS COL

A lot of cheapo guns have these DairyLea COL's and I have just recently bought some v3 ones

(for some future projects I'm looking at doing - stock JG ones are non ferrous/magnetic so they are going)

Pretty f*cking annoying I know you gotta tear it apart AGAIN coz of cheapo crappy alloy COL cutting corners etc....

Wish I could find that pic though - bastid photobucket - knew I should have uploaded it to imgur

but a worn COL still LOOKS OK but in fact is f*cked in operation

 

[Rogerborg]

Noted.  I'll buy another cutoff lever next time I'm doing an order, but, uh... as per my MP5K thread... it's just working OK now.  I couldn't find anything wrong, put it back together, it locked up, I took it apart, put it back together, it locked up, I took it apart, put it back together, and everything seems fine.

No idea.  Having seen the tiny tolerances in there, I'm guessing there was a too tight / too sloppy connection.  Prising the trigger contacts apart might have been the winner, it's the only real change I made.

And dammit, now I don't have an excuse to buy another AEG. <_<

Kind of want a long-long G36 DMR.

 

[Jez_Armstrong]

In unrelated news, while I was putting it all back together I managed to drop the whole gearbox and crack the end off my (plastic) air nozzle, so the box is currently still out of the gun awaiting a replacement nozzle.

is this the 21.03mm one?

 

[Rogerborg]

I measured what was left of it at ~21.5mm.

It's amazing how many eBay sellers don't bother listing the length of their nozzles.  Fortunately ak2m4 has a good comprehensive selection.

Actually, I've got it back together but haven't run any BBs through it yet.  Expect another "HALP! GUN IS NOT 2 FEED!!!" thread soon.

 

[Rogerborg]

HALP GUN DID NOT FEED!!!!!

Then I filed about 0.5mm off the nozzle and gun did feed.

I re-iterate: how do these things ever work?

 

[Prisce]

Engineering at its finest. What I do at work, the difference between a machine working and not can be 0.00025”. 

Never really comprehended until I started working with this sorta thing.

 

[Sitting Duck]

HALP GUN DID NOT FEED!!!!!

Then I filed about 0.5mm off the nozzle and gun did feed.

I re-iterate: how do these things ever work?

With fine tolerances on v2's compared to v3's

The v3 front of shell has a recess so tappet rests a smidge more forward (at front of cylinder head)

a tappet plate is about 3mm thick at front

On a v2 you get say 10mm movement,minus 3mm = 7mm total movement/retraction

On a v3 you get say 11mm movement,minus 3mm = 8mm total movement/retraction

With about 7mm on a v2,  a bb is 5.95mm, it needs best part of that extra 1mm left to seal at bucking

It is no wonder when stuff is not 101% precisely within that window or tolerance - YOU ARE F*CKED !!!

A v3 is a bit more forgiving imho as you got that extra 1mm travel/tolerance to begin with

Stuff still needs to be within the tolerance of the gun....

Krytac nozzles are 21.23mm or something

ICS M4 nozzles are same as ICS MP5 at about 20.5mm or something due to ICS hop unit

(stick in a SHS/RA nozzle on ICS and you really will get feed issues - you can  mod a reg M4 hop unit to fit)

PITA doesn't even begin to cover the bollox at times in these poxy ar$e toy guns

End of the day it is all about attention to detail - checking everything you replace with a digital caliper etc....

Surprised the SHS nozzle sits ok in the clear tappet plate with cut outs like Lonex tappets...

The SHS tappets do retract a bit further than other tappets but need to check it don't bottom out

(then f*ck about modding the SHS tappet up the front to fit in cyma v2 box (cyma v3's are normal))

before/after bollox....

These cyma v2's are a bit qwirky compared to G&G's

Now most v2's are about 3mm thick at the very front of the gearbox in front of cylinder head/tappet stop

D-Boys v2 are 2.65mm - a third of a mm more forward

This can cause other issues if a hop unit protrudes into the D-Boys box due to thinner wall

It will protrude inside and touch the tappet plate reducing the final return point (f*ck me)

so a small light file of hop unit to very slightly reduce the hop/nozzle entry port might be needed

(obviously check $hit first - but there is all sorts of bollox to watch out for on every box ffs)

I guess that is why certain techs use certain parts in certain boxes

to reduce the hassle of tolerance/modding to work correctly

If cyma fitted a ferrous COL than DairyLea COL you probably wouldn't have such a headache recently

Still - next time you will likely check EVERYTHING, boxes will take days than hours and hopefully.....

They run even smoother first time - or second time after a minor tweak/adjustment

I might open up a box soon after testing to say flip irregular spring or fit spring guide to boost fps if a touch low

And at that point I give the motor height a quick re-check in top half of box that it is exactly near as damn it

Not wanting to open her up over & over & over, but if I do I will check lots of $hit out at same time I mean

Piles of joyful $hit these toy guns

 

[Rogerborg]

Well, I should disclose that I put a Maple Leaf bucking in, which might have made that 0.5mm difference.

It's all good though, I always intended the CYMA to be a platform to fiddle with.  I've got enough backups to the backups now that I'm not going to be short of something to play with, and very nearly have an excuse to buy another primary - then leave the damn thing alone.

 

[Sitting Duck]

Well, I should disclose that I put a Maple Leaf bucking in, which might have made that 0.5mm difference.

It's all good though, I always intended the CYMA to be a platform to play with.  I've got enough backups to the backups now that I'm not going to be short of something to play with, and very nearly have an excuse to buy another primary - then leave the damn thing alone.

TBH - think I learnt a LOT more from the cyma box than just working on easier ones like G&G

So once you have overcome or understand some of the qwirks inside

well imho it will stand you in good stead for ensuring subsequent boxes work correctly

all a learning curve like most $hit in life, straight forward stuff is - ergh pretty straight forward

overcoming issues & problems, learning from our mistakes etc...

THAT is where we really attain a little more wisdom imho

ya doing fine, still works, my first SRC did not go back together at all - I made a right f*ck up of it

 

[Adolf Hamster]

HALP GUN DID NOT FEED!!!!!

Then I filed about 0.5mm off the nozzle and gun did feed.

I re-iterate: how do these things ever work?

could be worse, when doing a cleanup and general inspection of the m4 and f2000 for a big event i managed to get the nozzles mixed up.

que surprise when i geared up with the m4, went to test fire and nope, nothing, not a single bb

so i broke out the f2000 and while it fired, it fired horribly.

whats worse is that firstly i'm meant to be an engineer, and secondly the difference is 8mm in length so not exactly subtle  :lol:

 

[Rogerborg]

well imho it will stand you in good stead for ensuring subsequent boxes work correctly

all a learning curve like most $hit in life, straight forward stuff is - ergh pretty straight forward

overcoming issues & problems, learning from our mistakes etc...

THAT is where we really attain a little more wisdom imho

Agreed.  There's a school of thought in software engineering code monkeying that you should build one version to throw away, because you only really learn when you get things wrong, then figure out why and what to do about it.  I'm fully subscribed.

 

[Jez_Armstrong]

HALP GUN DID NOT FEED!!!!!

Then I filed about 0.5mm off the nozzle and gun did feed.

I re-iterate: how do these things ever work?

This is why i asked bout nozzle length, i have a list somewhere of what lengths particular guns use but unfortunately it also depends on the manufacturer as it can differ, there is one on ebay which says its an SHS gold and says its something like 17.88mm which is too short for an MP5, the normal MP5 (A4/A5/SD) uses a 20.40mm and the PDW and K both use a 21.03 so i dont know what manufacturer uses one that short

 

[Rogerborg]

Well, it was a corker of a question, because having filed down the 21.5mm nozzle enough so that it feeds reliably, it's now not air-sealing properly and the fps is down. 

Oh, and the box then started consistently stopping half way though a semi-auto cycle, with the nozzle retracted.  Flipping weird.  Then I took it apart and put it back together and it cycled fine. For now.  :wacko:

The cut off lever seems OK, but I'm not convinced that it's reliably pulling the trigger trolley back - that seems like it might be a little worn.

To try and resolve my problems, I've ordered:

A V2 new trigger switch (£3.26).

A 21.4mm nozzle (CYMA specific, £2.45)

A Jing Gong G36.  (don't tell the missus  )

 

[Sitting Duck]

G36 should sort it

 

[Rogerborg]

Mmm, and it's beige, therefore the best.

 

[Sitting Duck]

Oh, and the box then started stopping half way though a cycle, with the nozzle retracted.  Flipping weird.

The cut off lever seems OK, but I'm not convinced that it's reliably pulling the trigger trolley back - that seems like it might be a little worn.

was that on semi or auto did it stop mid cycle, the switch at back needs them twp prongs bridged by selector plate

(copper strip needs to make good contact and bridge them prongs - solder thick wire across them

or thin wire if using a mosfet - but if you replace with say SHS switch - don't have prongs as extra safety feature)

Not saying this deffo the issue, but it can cause iffy contact/cutting out if them prongs aren't bridged 101%

Think you need to replace COL - this seems to be major problem in beginning

to ignore this problem is like cleaning/replacing windscreen but keeping f*cked wiper blades

 

[Rogerborg]

was that on semi or auto did it stop mid cycle,

Semi only.  On auto it seems to stop reliably with the piston and nozzle both forwards.

Think you need to replace COL - this seems to be major problem in beginning

I agree and I likely will the next time I need anything else from ak2M4, but to my inexperienced eye, it looks unworn, undamaged and straight based on online images.

What does look slightly scuffed (again, no basis for comparison) is the corner on the back of the trigger trolley where the cutoff lever sits.  So for the sake of £3.26 (on top of my G36 order from TaiwanGun) I'm going to replace the trigger unit to see if that makes a difference (and to give me a 2nd unit for comparison).   Bearing in mind that this problem occurred suddenly when throwing in a daft lipo, I'd be more inclined to believe that it's down to instant damage to the trolley plastic than gradual wear on the cutoff lever.

However, I'd be happy to be wrong about that.   As above, I'll learn more by NOT fixing it and ruling things out than by throwing the right part in first time - I really am enjoying the process of figuring this out, at a couple of £££ a try.