Low FPS...Trying to locate airseal problem

[cjfield1129]

I have a CA33e (version 2 gearbox) with a long inner barrel (534mm). I am running a SP130 (new), torque gears, systema type0 cylinder, systema nozzle, Systema bearing piston head, Deepfire TN piston, bearing spring guide and stock cylinder head. When I chrono, the gun only shoots at 365 +/-. I have tested the airseal in the cylinder unit to find the problem. I try the "finger on the air nozzle" test and the airseal seems fine. The piston actually stops and I can only push it forward with extra force. From that test, it appears that I have great airseal. However, when I test the FPS, it is always around 365 w/ .20 and 325 with .25g. I have tried other springs, cylinder heads, changed o-rings, other piston heads, and changed buckings (guarder clear one). The only thing that has remained constant during my tests were the cylinder (systema type 0 ribbed design), the piston (Deepfire full metal teeth piston), and the nozzle (systema nozzle with o-ring inside). All appear fine. Could anyone offer any possible advise or any better airseal tricks. I have a Hurrican Bore-up cylinder set on order. I liked the aluminum (yellow) two-oring cylinder head design better than systema's steel cylinder head as it would not have as much impact on the mechbox. I am stuck an could use some advice. Thank you

[MadMax]

remove the mag and drop a single bb into the bb inlet. Fire the gun with your hand placed over the magwell. If you feel a strong puff air, I suspect that your nozzle is not sealing in the breech assembly. It might not be pushing all the way forward before the breech is pressurized due to a fitment issue or a nozzle that's a bit short.

A small bit of leakage here is kind of normal, but if you have a lot of blowby, it could be the cause of your troubles.

How clean is your barrel?

[firemachine69]

Doesn't the heavier piston combined with super torque-up gears affect performance?

[damage]

change your piston head O-ring. I recommend the stock TM O-ring.

[CDN_Stalker]

I put in an upgraded Systema cylinder and head/nozzle into a G&P SPR and it had low fps as well. I figured since there was the stock G&P metal hop up unit still in there, there was a mis-matching and was leaking. We installed a Systema hop up unit in, with a Prometheus 6.03mm tightbore, the leaking went away.

Point: Either change your hop up unit to the same brand as your cylinder head/nozzle, or put your original nozzle and/or cylinder head back on and see if it improves. Your 130 spring should be putting out around 420fps.

Hey, try it, might help, might not.

[wkma7six]

I've had problems with CA's version 2 mechbox in my CQB-R. Had the same FPS results as you did and I had several reputable shops take a look at it while I took the gun apart and retested in front of them. I was told to replace the hop up unit, hop up bucking, and even the piston o-ring. As in your case, everything sealed well and worked properly, but there was an obvious air seal problem because there is no way an SP120 would be shooting at 365 FPS with a 363 mm M4 inner barrel. So one last thing that I don't recall anyone ever mentioning on any airsoft forum is to check the 2 mating halves of the gearbox itself (where the hop up unit seats against). Mine was about 0.5 mm offset from both sides. I proceeded to file the longer side flush with the shorter until I got a flush even surface. You'll have to do this carefully with a small flat file and make sure the the 2 halves of the mechbox don't take on a domed shape while filing.
I then proceeded to put the gun back together and applied a little silicone grease (between the mechbox and hop up unit) to make any airleaks a little more obvious after testing and for a final teardown inspection. My results were 412 FPS after resurfacing the mechbox case. That's a huge gain from 365!!
I also have a Marui based gun where the mechbox is perfectly flat at the hop up mating surface. Quality control is so much better with the Japanese made products, but with a little work, CA stuff can be made to perform just as well with a reinforced gearbox.
Sorry for a long first post, but I hope this helps many of you out with those "low FPS" problems with near perfect working parts.

[GraveTech]

An alternative to filing would be to actually apply a seal around the gearbox. Just make sure you don't apply to much otherwise it could make for one hell of a cleanup job.

[wkma7six]

Originally Posted by GraveTech

An alternative to filing would be to actually apply a seal around the gearbox. Just make sure you don't apply to much otherwise it could make for one hell of a cleanup job.

I have tried that with 3M friction tape and cut out enough material to use as a gasket. Didn't want to use a more permanent sealant (RTV, caulking, or what ever one can come up with) as these leave too much residue to clean up for the next tear down. The friction tape worked well for awhile, but I had to carefully reposition everytime and it eventually tears and wears out. Friction tape leaves no sticky residue and is cloth-like material (like duct tape) but with a butyl rubber-like material coating on both sides.
Soooo.......a permanent fix to the actual problem seemed viable rather than putting on a "band aid"