M4 Loss of power

[MultipleParadox]

Hi there, I've been wanting to post this a while ago and never had time, so here goes:

Gamed my KA M4 3 times so far, and at each games I had a significant decrease of fps on the chrono.

Firstly, KingArms advertise my model to be ~385fps. When I received it, it was obviously much much lower than that, probably around the 300-320 mark (pure speculations based on my small experience and comparing to my M93RII @ 345fps)

So I didn't make fuss of it, bought a PDI 140% and dropped it in. Went to my first game, and chorno'd at about 350-360 fps.

Second game, it dropped at about 340fps.

Third game, 312-315fps. So I'm at the point where I say "WTF?"

--

Yesterday I gave it a good cleanup from the sand of the field, and took the opportunity to check up on my gearbox. Everything seems alright, no wear on the gears, etc. However I noticed the O-Ring on the piston head is really really loose. I'm pretty sure it's not normal but I could be wrong. I though it should be tight around it but it's not.

When I place my finger on the nozzle and push the piston in the cylinder, I feel practically no resistance at all. Again, I expected it to be hard to push, right?

Also, the o-rings around my hopup seem to be a bit small, not sure if they should be that small. If anyone could provide a photo or something so I could compare?


Thanks!

[Reckless]

the oring should actually be fairly loose.. almost falling off some piston heads... take the oring off the piston head, and stretch it over the cylinder you are using. leave it for awhile so it stretchs out.

then put it on the piston head, lube it , and carefully put it back together (I say carefully cause if your not careful the edge of the cylinder can cut the oring) .. .slide it up and down a few times to distribute lube and such... then redo the compression test (finger over cylinder head tube) it should stop dead wayyyyy before the end of the travel... if you have ports basically right past the ports. being loose on the piston head is good, it'll let it travel backwards with less effort, the ports in the piston head will push the oring out to seal to the cylinder ... gives you seal on compression, and little to no seal/effort on the pull back

[MultipleParadox]

I think I get it.
So basically I should stretch to ORing the most possible?

I'll give it a try when I have a chance

Otherwise, is there any other parts I should check for a problem in case I'm not looking at the right place? I'm pretty sure the problem lies in there as I don't seem to get any compression like I said in the OP but ...

Thanks

[audi_bhoy]

FPS decrease can also be due to the gun being stored with the spring half compressed, if your last shot was in full auto, there are chances that the piston is pulled half the way back, and this is not good for long time storage. When you finish playing with it, be sure to shoot at leats one time in semi, so the piston can sit all the way in the front, preventing this.

[MultipleParadox]

Yeah I thought about this issue. While I can't be 100% sure it was not the case, it's not likely as I took the habit to fire a single shot or two after each game in case a BB's still in the barrel/hopup...

And when I opened it yesterday the spring seemed to have all its strength and all.

Thanks for the insight though

[Reckless]

Originally Posted by MultipleParadox

I think I get it.
So basically I should stretch to ORing the most possible?

I'll give it a try when I have a chance

Otherwise, is there any other parts I should check for a problem in case I'm not looking at the right place? I'm pretty sure the problem lies in there as I don't seem to get any compression like I said in the OP but ...

Thanks

not exactly as much as you can. just stretch it over the cylinder your using and let it sit for awhile ... I've seen some say to go as far as heating the oring a little with a lighter or heatgun... but I never had too, or attempted it... with the stretch method, I generally only see 4-6 fps variance from the guns I've built ... this ofcourse depends on the other sealing places too like the cylinder head to cylinder, the nozzle to cylinder head tube, and the nozzle too hop bucking lips with in the hop up chamber...

the 2 main, are the piston oring, and the nozzle to hop rubber/chamber

[MultipleParadox]

K,
I'll investigate a bit more with these info at hand when I have a chance and will come back as needed

Thank you guys

If anyone wants to pitch in with more insight in the meantime, feel free to do so!

[MultipleParadox]

Bak again

So yesterday I disassembled the whole thing again.
Did stretch the o-ring a bit as per Reckless suggestion. Now I wasn't too sure how long "a while" is, about the time to leave the o-ring stretched over the cylinder, I left it there for maybe 10-15 minutes?

It did help a lot, as I could now feel the piston compressing the air. It was still possible to push the piston completely into the cylinder with some force. I have a small leak on the nozzle and there seems to be a small one between the cylinder/cylinder head.

I know using a nozzle with a o-ring (mine don't seem to have one) would solve this one. I'm not sure what to do about the cylinder head though :S

Anyhow, I reassembled the whole thing and did a quick "poor man's chrono" test.
Result: Didn't even go through the side. According to the guide, going through one side is between 290-310 fps. So it probably means it's even less powerful than at my last game where it was chrono'd ~312 fps

Remember I have a PDI 140% spring in there...
I doubt the small leaks I get could be responsible, especially after having stretching the o-ring to be able to compress, while before it was completely loose and wouldn't do shit...

Help?! Not sure what to do from there.

[Spawn28]

Your either getting a really bad seal at the hopup or possibly even a cracked Cylinder head KWA's are netorious for this. If the nozzle of the cylinder head spins or has any movement or give its broken. If your getting a leak between the cylinder head and the cylinder check the O-rings on the cylinder head and relube you might have a deformed cylinder or head.

[MultipleParadox]

Note: KA as in King Arms, not KWA. Not that it really matters anyways

By "head's nozzle" you mean the brass tube on wich the plastic loading nozzle sits?
If it's the case, I don't think it's broken then, no movement or anything from there

I'll double check the head's o-ring.

As for the hopup seal:

as stated in the OP, I think I might have an issue there as I feel the o-rings are quite thin and not sure if it's normal. I'll take a pic tonight and post it. If anyone could do the same with theirs it'd be great for comparison purposes.

Thanks for the input, I hope I'll be able to find the problem... :/