compression issues

n1ckh

n1ckh

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I've got the cm16 raider which has a few upgrades in the box running 8.4v NiMH

SHS O-ring nozzle

SHS double O-ring cylinder head

SHS 14 metal tooth piston

Super shooter double O-ring piston head

M100 spring

Everything else is basicly standard, its shooting between 265 & 270 fps but want it shooting around 300 for CQB sites as that's what its for now

Airseal around hop up, nozzle & cylinder head are fine so I think the air is escaping around the piston head, ive put grease in the cylinder but no matter what I do, it keeps happening, i dont think the cylinder is out of shape and its the right way round in the box

Not sure what to do now ??

 
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Had/have similar issue with my cm16. I have checked my air seal, and it's perfect.

Only thing I can think is its the nozzle to hop seal that's the issue.

 
I've done the compression test with just the cylinder head in the cylinder, put my thumb over it and pushed the piston forward with very little resistance

I'm going to try the Teflon mod just to rule that out but I'm fairly certain its the cylinder

I only installed the piston head yesterday and hasn't been skirmished yet,

could the piston itself be the problem ??

 
You can't really test it that way. You can have good airseal if you can't push the piston in, but you can have good airseal if there is not much resistance too. It still can seal on the high speed it normally moves.

There are also few cases where you have good airseal with this manual way, but "too good" on high speed. There can be too much space between the piston head and the cylinder and on high speed the O ring is pushed so much into it, that it slows down the piston. Oh, how I hated that gun until I found that this is the problem. :)

So usually if you have good airseal manually, it will be good in normal use, but not always. Try different O rings or piston heads or cylinders.

And always use a tiny amount of light grease like silicone in the cylinder.

 
Thanks guys for your replys

I bought a pack of piston head O-rings just in case it was the ring as its been in there since I bought it 8 months ago so that's been changed, I might try the floss idea under the ring and see what that does

Think I'll try a different cylinder next just to see how it goes or just leave as is

Good thing is I have my g36 I use outdoors aswell

 
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Don't put floss or anything under the O ring of the piston head.

You can put it under the o ring of the cylinder head if it is too loose.

 
Ahhh ok

Thanks for the warning :)

 
yes no tape or even extra thin o-ring either under piston's main o-ring

I thought that might work better but f*cks it all up with another tiny thin ring/tape

is weird but the piston & o-ring design is to allow air into chamber from rear as piston is pulled back

(or it can create a vaccum if sealed completely)

then those same little piston vents/holes that filled with air - now help to seal the o-ring/cylinder as piston is released

(quite clever really)

Your Double o-ring piston should work well but all the same never hurts to try another o-ring or two and compare so...

Try another couple of o-ring's and wrap ptfe tape on cylinder head say 3 full turns maybe until it is tighter to fit to cylinder

test out the compression test again and again keep at it - when you get these all good the seal is like water in there

but still hardly any drag pulling piston back - a £10 or £15 cylinder head may work better with a crappy cheap/used piston

or vice versa - is weird but often what I thought was gonna be a very good seal turns out not so brilliant as I thought

and when you get a great seal like I said you really notice it on compression test - even with a cheap used bit in there.....

Even different o-rings - some used ones can work better - it is weird but some $hit works really great and some not so....

if after all that your piston is making good seal on cylinder head/nozzle then my guess is hop area......

check if there is a spring at front of hop or look to fit one or card or thin small o-ring to push hop against nozzle

change bucking if still no joy - if still crap try another hop - but shouldn't come to this I feel

if box/nozzle is good - then it ain't the box - but what's after it I reckon is where you might be losing it

poxy guns

 
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Cheers duck

Its weird as I've done the same mods to my 36c & and my sons m4 and there fine :/

I've got 2 hop units, the original and upgrade one that both have springs on, I bought a new green buck as I put that one in my g36c

As its for CQB and a back up, I suppose its ok but I'll defo do what you guys suggested

 
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it might also be spring but I really doubt it - you replaced stock spring with shs m100 a little while ago

that should put you at 330-340-ish, now say 6 months later you should still be hitting 300-ish

all springs lose tension a little - more so if spring parked badly pre-cocked

should not have lost so much ooomphh unless serious wear n tear/bad spring park etc....

get seals sorted, then check hop malarky that should improve fps me thinks

 
It was a mates spring so he gave it to me when he downgraded his but it was virtually brand new

I have noticed most of the time the spring has stopped in the compressed position, shame G&G dont have a spring release button like my mates ICS

I'll be getting PTFE tape later after I've taken my daughter home then I'll have some peace to have another go at it

 
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when you do the compression test get your lips near the cylinder head. You will feel it if the leak is there.

 
Is your CM16 the L or S model? My S model came with low FPS but when I tried it with an M4 length inner in it it jumped straight up to 330 fps. G&G use the same gearbox in both the L & S models and the much shorter inner barrel in the S model is what gives the lower fps. If you have the S length then an M110 spring will probably put you into the range you want.

 
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