Gradually Decreasing FPS

[Spike]

I have my buddy's M4 here and this is how it's shooting:

380>380>375>340>280>200>80>jam

I checked the compression, it's good in the piston/head/etc and between the nozzle and hop up.

My theories are:

1 - 11.1v lipo is spinning the gears too fast and it's catching prematurely
2 - the first piston tooth is removed, causing 1 - to be more intensified

Any other opinions or theories? I'm going to try an 8.4v battery tomorrow and a new piston if that doesn't work.

[Boyso]

I'd like to know as well, my first AEG had the same symptoms. Never found out why.

[MaciekA]

Does it exhibit this pattern when firing without the upper receiver or hopup attached? How about without BBs loaded?

[Spike]

How would I be able to test FPS with no BBs loaded...?

[ThunderCactus]

lol hilarious
Try it with a lower voltage battery to see if the LiPo is the problem, although high speed usually only causes misfeeding and jams
Is the hopup on? If it's off, and BB's are rolling down the barrel, it would definitely cause that problem. Is the hopup too tight? And is it changing it's position as your fire?
Try re-installing the hop rubber, or even upgrading it
And kind of unorthodox but it's a common issue in G&Ps, load 1 BB at a time, and use a flathead screwdriver up inside the mag well to hold the hop chamber back against the mechbox. Sometimes the hop chamber is too far forward and needs either shims or a heavy spring to hold it back against the mechbox.

[Spike]

It's a KWA 2 piece hop up (gen 1) so I doubt that the unit is too far forward, I don't think it would feed at all if that was the case.

i'll try turning the hop up on more when I test it again, thanks for that.

[MaciekA]

I know it sounded a bit weird, but my suggestion was intended to verify everything is cycling okay in your gearbox. If something is catching due to over cycling, your fps wouldn't matter. You could probably find ways to simulate a bit of back-pressure into the gearbox without having the hopup involved.

Having seen extremely similar issues on a G&P lately, I would have thought the hopup was having problems (like ThunderCactus said, too far forward, or perhaps a loosey goosey bucking). I've resolved two jamming problems recently by simply securing or pushing (with a spring, o-ring, whatever is available) hopups back as little as 1mm.

[MultipleParadox]

I had the same kind of issue with a Brand New King Arms M4
Compression was part of the problem, but I also the spur gear (not sure if I'm mixing up terms, the bottom flat one) was ever so slightly crooked (barely visible).

Changed the gear set and compression parts, problem solved

[Spike]

SO here's an update:

Tried a 9.6v battery, now the FPS stays consistant. The new issue is that it's only shooting 260. The sorbo pad that was in the gun had fell off and was mangled up in the piston, so I removed it, but still the low FPS. There's no compression leak between the piston and cilyndar or the nozzle to the hop up.

I'm thinking of just getting a modify cylinder set of it, do you think that would fix the issues? Gen 1 KWA

[HKGhost]

I've worked on a KWA M4 that had the exact problem. The top part of the two piece hop up is the cause. The spring that's suppose to push the hop up towards the gearbox could be too strong or too week. The KWA hop up system is kinda stubborn and likes to work in a specific way. Also try removing the little spring and see if that helps. You can try replacing that small spring for a larger one that goes around the inner barrel. I fixed the problem by removing the little spring. Hope that helps you.

[Spike]

Tried friggin' aound with the hop up upper portion...got an increase of about 20 fps, but no luck otherwise. Tried removing the spring, tried replacing the spring, and tried manually pressing the hop up unit against the gearbox. Turning the gun upside down and trying that compression test yielded no issues... the BBs hardly moved.

I'm out of ideas on this.

[R.I.T.Z]

try a different motor?

[cetane]

I had issues with a v2 gearbox that it was shooting about 90fps low and would load the motor to point of binding & blowing fuse. Ended up being an issue with the piston. It was just slightly wide on the guide rails and would drag on them. If it stopped mid stroke off an auto burst it would not have enough torque to pull it and would pop the fuse. It functioned normally for the original owner with no issues for like 3 years and acted up for me after about a month of owning it. Ended up filing down the piston guide rails on piston a bit both sides so it freely traveled in Mechbox. Once done that it was working great. Shooting constant 350fps +/- 1fps. Dunno if that is what be wrong with the OP rifle but could be...?

$0.02

[MaciekA]

I was working on a gun today and putting a new Modify hopup into it. Was firing 300 when it should have been firing about 360 to 370fps. Tried some of the things you did and suggested above, different hopup springs, tried 3 different buckings, manually pushing and holding the hopup (via holding the inner barrel and pushing it) against the gearbox, etc. None of these things fixed the issue. I did also observe a 10-20fps improvement if I held the hopup unit very tightly against the gearbox, but it would go away as soon as I let go.

Then I tried something I've been doing to my AEGs lately with some luck, which is to wrap the hopup tightly in a bunch of Teflon tape, and then put an o-ring on the end of the hopup, like this:



Note: Wrapping the Teflon tape continually around the rest of the barrel like in the picture above isn't really necessary for this to work (it can cause the tape to bunch up against the hopup if your outer barrel isn't big enough to hold it all), but I found it can help secure the inner against the outer and prevent it from rattling or wobbling around.

After the above modification, the rifle in question is firing a consistent 360fps through the chrono at 20rps with no jamming issues. Perhaps you could try something like this. Shot in the dark.

[Spike]

I think I will try the tape, once I manage to get my hands on some.

I replaced the piston in it, so I don't think it's that.