Good airseal but low FPS? (AK)
 

So I'm working on my AK103 gearbox (the gearbox is an ICS shell/gears/etc but with Cyma piston/piston head/cylinder head. ICS nozzle).

Opened her up, everything looks great, good airseal (simple finger on nozzle, pump the piston test gives good compression) but it chronographs about 170-200fps with .2gs.

Thought it could be the Cyma hopup and barrel, but replacing it with a TM unit and tightbore barrel made no difference at all, even with the hopup all the way off.

Thus I turn to you, the more experienced gun techs on this forum for help.

Any tips?

Thanks,
Cheers,
Alex

A little test for you to try;
When completely assembled, reach up into the mag well and see if you can wiggle around the hop up unit, specifically forwards and back. I have found that you may have to sometimes shim the hop up unit back (towards the mech box) because the body housing, mech box, and hop up do not align properly (a real problem with some early G&P AR metal body kits). If you find movement, you have probably found the problem. Otherwise you will have to continue your troubleshooting.

Also, make sure that you have lightly lubed the cylinder.

Best of luck;
SHA DO

Sha Do is probably right but just to play devil's advocate...are you sure you have a good spring? Is it definitely a compression problem?

Yeah. Try replacing the spring?

If you can, I'd recommend getting the pro-win hop-up for AK's (Jugglez usually carries them), it screws right onto your mechbox (As long as the ICS is like the tm one)

Thanks for the tips guys. Lets see:

Hopup is rock solid.
As I said in the first post compression is GOOD. It is not an issue. The spring seems fine too, it is quite stiff, stiffer then a stock spring that's for sure (it's a Guarder SP110 as far as I know).

I heard airseal issues with Prowin hopup, as well as fitment issues before. Stock Cyma hopup's work great for me. And as I said I tried a TM one (it's installed right now)

Thanks,
Cheers,
Alex

Compression may in fact not be good. A slow speed test with your finger does not always replicate good high-speed flaring of the piston head o-ring against the cylinder wall. I've found that when using full cylinders, use a ported piston head. If the CYMA head is a ported head, try a different o-ring or replace the head.

You will also be able to tell if your comrression is good by dry-firing the fully assembled gun and seeing how much air comes down the barrel (use a tissue paper at the muzzle) and see if any air is leaking out the feed nozzle at the hop-up.

Thanks Mcguyver but it wasn't any of that. I talked to Thundercactus on these boards, and he tracked it down in no time. ICS shell needs to go, doesn't mesh well with the hopup. After unscrewing the hopup unit and pressing it up against the mechbox it fires 400fps on the dot.

Thanks everyone! Now I either gotta find a way to shim the hopup (not likely) or need a new shell.

Cheers,
Alex

What do you mean shim hop up? Do you mean to add teflon tape to prevent air leak or modification of hopup another way?

are you sure you have the spring installed in the right direction?

You can try cutting some duct tape into small squares, then stacking them on the inside of the upper receiver where the hop up unit sits against it, effectivly shimming the hop up back into the mech box.
This should work until you can get the right placement, then use JB weld and build up the area, then file.
That's what we did to my G&P SR. However, I am unsure how the AK's hop up sit's in comparison to the body, so this may be pointless.

SHA DO

Easier to open up the hop-up mounting screws into an eliptical shape and slide the whole hop-up back. Then screw it down again. A Dremel and 5 minutes should be more than plenty.

If your concened about it sliding forward again, put something in the gap and secure it with a gearbox shim under the screw.

Seems like an easy fix to me and will cost you nothing.

Well what I mean by shim the hopup is I need to insert something inbetween the hopup and the front end to make it sit in a different position. Thing is it's not only supposed to sit back, but also possibly on the side. It wasn't easy for it to get it to seal and get high FPS, I really had to fiddle with it. Don't think I'll be able to replicate that position properly for it to work perfect.

Dropping in a Kraken mechbox had it working perfect.

I'll just get a new shell.

You don't read do you.

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