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Discussion Starter · #1 ·
So I decided to do some upgrades on a Cybergun CQB-R.
I've actually opened it up a few times at this point.
I'm thinking the piston may be tight in the gearbox.
After rebuild, it starts around 330fps (which seems a little low for a M115) and after 5-10 shots, drops down to 280, with fluxuations down to 200 and even had a few bbs roll out of the barrel.
Original nozzle was 21.5mm
I've tried 21.5 and 21.25mm nozzles
Tested airseal with and without nozzle, as well as with tappet plate cocked 90*
Air seal is solid.
Passes tissue paper test.
Tested with hop in several different positions.
Tried different hop chambers, barrels and packings/nubs.
Shimming is good.
Tried different batteries just in case. 2s and 3s lipos
Tried different combinations of cylinders and heads.
SHS 16:1 gearset
M105 and M115 springs
SHS 15T Piston with POM head
CYMA Platinum mosfet, which I believe has a little precocking built in.

Is it possible the piston is tight in the gearbox?
That's the only thing I can think of at this point.
 

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Can also check with the gearbox assembled if you have a open tipped nozzle - pull the upper off and slip a long ~3mm allan key into the nozzle to see how hard it is to push the piston back. Should require a firm push but move smoothly. Be careful you push straight in, don't want to bend the nozzle guide tube. May need to pull the motor if it's parked mid cycle.

FPS deviation like that could also be due to a weak tappet spring, midcap syndrome / sloppy nozzle, to weak or to much hop tension, damaged or bad seals on piston or cylinder head...
 

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Discussion Starter · #4 ·
Thanks for all the tips. I'll mess around with it and report back.
I will try all the non-invasive stuff first.
I did try a few different magazines. Some half loaded some fully loaded with the same results.
I forgot to mention, there is a possibility the tappet plate could possibly be binding up.
It occasionally seemed like it was maybe locked up when I would try to push the nozzle in by hand.
I do have other plates and springs on hand that I could try.
I also have a selection of Pistons if it ends up being that the one that's in there is too tight.
 

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Discussion Starter · #5 ·
Set your sector gear in a way so that it doesn't catch the piston rack and see how well you can shift the piston around, if at all. Should be relatively loose on the track, but not super wobbly.
Can also check with the gearbox assembled if you have a open tipped nozzle - pull the upper off and slip a long ~3mm allan key into the nozzle to see how hard it is to push the piston back. Should require a firm push but move smoothly. Be careful you push straight in, don't want to bend the nozzle guide tube. May need to pull the motor if it's parked mid cycle.

FPS deviation like that could also be due to a weak tappet spring, midcap syndrome / sloppy nozzle, to weak or to much hop tension, damaged or bad seals on piston or cylinder head...
Ok. I got home from my 9-5 and had 1 hour to spare, so I dove in.

I removed the motor and pushed in on the piston head through the nozzle with a long allen key. It bound up near the end of the pistons travel.

Ripped the gearbox open, removed everything but the compression parts.
Screwed the box back together.

The piston started to get friction about half way through it's travel.
Also tried it without the cylinder, just to be sure. Still friction.

Measured the old piston vs new shs.
Od of the rails
Old 21.9mm
Shs 21.46mm

Id between rails
Old 18 36mm
Shs 19.03mm

I didn't have time to break out all the proper tools to narrow between the slots of the shs piston, so I swapped the POM head to the old piston and reinstalled everything.

It all works good now. About 360fps with m115 spring &5fps deviation. Need to fine tune that.

Thanks again for the help!
 

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Edit:just realized this was solved after posting my bad.

Typically I find horrible inconsistencies are due to two possible causes. One being mags, late chambered rounds always cause issues like this, a bucking with the lips protruding into the hop up feed tube will cause this or any other feeding obstruction.

Second is the piston head o ring and piston head. The o ring needs to be wide enough to drag and seal against the piston head. Anything that prevents that will cause the sealing to be randomly delayed resulting in fps issues.

If you have any sector chip, remove it. Nozzle open time isn't as important as nozzle closing time for most builds.

Pistons and tappets should always be tested for fitment before installing any other components into the shell. You'd be surprised how many pistons won't fit in some shells, tappets too.

Swapping parts like crazy isn't the right thing to do, it just adds more mental pain and variables, it's best to take a detailed look at all installed parts to figure out which one is malfunctioning.

Heck the other day I had a zci piston with a warped front face cause a piston head to be crooked and bind. Ruined a bunch of parts.
 
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