Very Low FPS

[Coma]

For some reason, my AK is only shooting at 120 fps. Pre-fuck up velocity was around 350. Schwag suggested that perhaps my cylinder port was backwards, but my cylinder is not ported....

I just put in a replacement hop-up on Saturday night. Anyone following my progress in the past (I'm looking at you, ancorp!) knows I've been having issues with the Prowin hop-up unit I installed. I replaced that one with the ECHO1 unit. Being a TM copy, and since the TM unit worked fine, so should this one. And it does, feeding flawlessly. At least with the hop-up all the way in the 'off' position.

I've looked at my internals, as have others on Sunday, and nothing seems amiss. My battery was freshly charged. There was no air leak between the nozzle and the hop-up. Does any one have any other ideas for me, or perhaps even a solution?

Much appreciated;
Coma

[Spa]

use the cylinder head + cylinder + piston and piston head, plug the cylinder head nozzle and start pumping with the piston to see if compresses air (you wont be able to push the piston down after a couple pumps to warm it up)

If thats the issue, gotta work around that, if not i guess we can work from there

[Greylocks]

Check if the piston head has not separated from the piston itself, then check the nozzle and hopup areas.

[hutch1970]

Piston o-ring?

[Greylocks]

That too, thankfully you can buy replacement o-rings of the right size at any hardware store in the plumbing section.

[Syn]

So it passed your compression test?
If it failed, try the SPARTAN O-ring mod then try the compression test again.
Also check that there is enough cylinder lube.

THen I would look for other courses along the bb path.

[Firas]

i just installed my pro win hop up and my fps is very low like yours. I will try what has been suggested and get back with my findings

[Firas]

so it has to do with the hop up, because when i put in my old hop up/barrel it shoots fine. There doesn't seem to be anything wrong with the new hop up its self. Any ideas?

[ILLusion]

It's a very well known issue that the Pro-Win hop up rubber sleeve is useless.

[Firas]

i tried it with another rubber and it did the same thing. I think it has something to do with how it is sitting with the mech box because when I hold the barrel/hop up instead of having it mounted on the body it shoots at a decent fps. When everything is mounted it seems to have the hop up pulling to much upwards and I am guessing there is a lot of air escaping out the bottom?

[CDN_Stalker]

Cycle the mechbox until the nozzle sits full forward. Shine a flashlight into the magwell (loading tube) and look down the barrel to see if any light is coming through. That's what I tend to do when I suspect sealing problems and it works very well.

[Firas]

aaah very nice idea will try it
EDIT: when I do that the nozzle only sits about half ways past the loading part of the hop up. So when I shine a light in there it completely lights up the barrel

[CDN_Stalker]

Half way is the farthest forward you can get it? What upgrade parts do you have in there? You should be able to get it full forward, I find quick full auto bursts makes it come to rest there once in a while.

[Firas]

no upgrades, just new bushings. i will try longer full auto bursts
Edit: tried longer full auto and it never goes past half ways.

[CDN_Stalker]

Originally Posted by Firas

no upgrades, just new bushings. i will try longer full auto bursts
Edit: tried longer full auto and it never goes past half ways.

That is odd, seems every half dozen cycles the nozzle rests full forward. How is the airpressure coming out the front of the barrel compared to what comes out of the nozzle on the mechbox itself? Makes no sense if all you had replaced was the bushings.

Hey, does the nozzle move at all when you dry fire it? Hold it upside down and watch the nozzle move. Am wodering if the tappet plate is not working right, or if the nozzle isn't installed properly on the tappet.