Cyma SVD fps all over the place

[Austeyr]

Took my Cyma SVD out Sunday and noticed every now and then my shots would veer off randomly. Today I figured I’d chrono it and the variations are all over the place. 
 

All readings are on a 0.28g BB

313.4

386.6

392.1

317.6

312.6

318.8

316.0

315.7

318.1

386.1

319.4

319.3

323.3

322.6

299.8

317.5

324.3

304.1

320.2

325.8

324.6

304.0

400.2

I fitted the following parts:

FPS Airsoft Aluminium Cylinder Head for CYMA SVD

FPS Airsoft Ergal Air Nozzle Inner O-Ring for CYMA Dragunov SVD
CYMA SVD/SVU Piston and Head Set

ASG Ultimate Mil-Spec AEG Spring (M135)

Maple Leaf MR Yellow + Omega Nub
 

When fitting the cylinder head I had to remove a tiny tiny part of the oring edge to get it to go into the cylinder, but the air seal passed all of the tests…

 

What could be causing the low fps (In terms of the spring power) and the random jumps in power?

I’m thinking of buying the Cyma cylinder head and nozzle but the cylinder head has a 21 day wait time 

[Adolf Hamster]

sounds like this might be a case of the ml longer feedlips and the gun saying no.

 

i'd try swapping the bucking back to stock, if that fixes it then it's either trying to sand down the feed lips a smidge (which sadly often makes it worse unless you can get it real clean and square) or a different bucking.

 

otherwise, time to look at the nozzle/cylinder head.

[Austeyr]

34 minutes ago, Adolf Hamster said:

sounds like this might be a case of the ml longer feedlips and the gun saying no.

 

i'd try swapping the bucking back to stock, if that fixes it then it's either trying to sand down the feed lips a smidge (which sadly often makes it worse unless you can get it real clean and square) or a different bucking.

 

otherwise, time to look at the nozzle/cylinder head.

Thanks, I’ll give that a try! 

[The_Lord_Poncho]

In addition to the above, perhaps double check that the hop unit is hard back against the gearbox too. I'm unfamiliar with the SVD, but presuming its the same rough idea as many other designs, adding a few o-rings onto the inner barrel at the front of the hop unit can work to push it back nice and snug. You mention that some shots are veering off - if you are using your original inner barrel it is possible that the MR Hop and the Omega combo are too big for the window cut into the barrel, and you may be getting irregular pressure/hop as a result.

[Austeyr]

Changed the bucking over to the original black one, still having FPS spikes albeit a LOT hotter, one of the shots chrono'd at 468 (On a .28g BB 😳)

 

The hop unit is bolted to the back of the barrel so sits firmly against the gearbox, and has a large spring also pushing it against the gearbox too. I noticed the hop unit itself has some wobble on the top wheel which seems to be a common thing with these, so I'm going to try the mod shown here; 

 

 

[Hudson]

Are you chronoing with or without hop up?

And does the chrono have fresh batteries?

 

 

I've had my own struggles with the Cyma SVD, that took a long time to get anywhere near resolving unfortunately!

[Austeyr]

19 minutes ago, Hudson said:

Are you chronoing with or without hop up?

 

With it set roughly in the middle, which is where it was (Again, roughly) firing straight when using it at the game.

 

19 minutes ago, Hudson said:

And does the chrono have fresh batteries?

 

The batteries are old, but it's not giving any issues with other replicas I've tested through it recently

 

20 minutes ago, Hudson said:

I've had my own struggles with the Cyma SVD, that took a long time to get anywhere near resolving unfortunately!

😢 I knew I should have hunted for a RS 😂

[Adolf Hamster]

1 hour ago, The_Lord_Poncho said:

In addition to the above, perhaps double check that the hop unit is hard back against the gearbox too. I'm unfamiliar with the SVD, but presuming its the same rough idea as many other designs, adding a few o-rings onto the inner barrel at the front of the hop unit can work to push it back nice and snug.

 

ahh yes, so essential i forget to mention it.

 

1 hour ago, Austeyr said:

Changed the bucking over to the original black one, still having FPS spikes albeit a LOT hotter, one of the shots chrono'd at 468 (On a .28g BB 😳)

 

well that's partially good, means the issue isn't necessarily the ml fee lip length.

 

one thing to contemplate is ammo, if it's a bit sticky might give what you see. the number of these sorts of issues i've "fixed" by handing a mag filled with geoffs to the complainant.........

 

otherwise next candidate to look at is the nozzle/cylinder head, one assumes they seal well to each other (worth checking with the thumb test) but possible that it isn't working when feeding, but sealing against the hop it just takes .01mm too short to get intermittent seal.

[Austeyr]

19 minutes ago, Adolf Hamster said:

one thing to contemplate is ammo

 

The BB's are probably low quality, will try some BLS .28g as they've got a good rep I believe

 

19 minutes ago, Adolf Hamster said:

one assumes they seal well to each other

 

Checking air seal it was solid with the thumb test, I think I'll still get the cyma parts ordered as they're OEM+ so should fit better (Hopefully!)

[Hudson]

I struggled to get any decent performance out of mine with a maple leaf MR or Macaron variant.

One of the very basic original 60º Maple leafs seemed to work okay for a while, but not having a range to test it out on, it was a long slow process of 'build up and set for a safe power with no hop' then take it out to a game, set the hop and then test it.

 

Often resulting in being massively under the power limit once I got my hop set for my chosen weight (either 0.32 or 0.36) and then requiring me to come back home, disassemble the gearbox to install a new spring to suit. (Later on I bought an SVD-S that had a quick change spring I could change without taking the gun apart. So much easier!)

 

By now, over a year and a half after originally getting it, I ended up with a 650mm 6.02 tightbore (maybe ZCI, I can't remember?) with a smaller window, and just a basic design bucking. (Laylax 60º blue) That seemed to give a reliable, fairly consistent performance that I just wasn't getting with any of the fancier buckings.

 

Gearbox wise, I got one of the FPS nozzles with o-rings, some piston o rings from @ak2m4 and a guarder M110 that ended up giving me around 1.64J once hop was set.

[Austeyr]

12 minutes ago, Hudson said:

I struggled to get any decent performance out of mine with a maple leaf MR or Macaron variant.

One of the very basic original 60º Maple leafs seemed to work okay for a while, but not having a range to test it out on, it was a long slow process of 'build up and set for a safe power with no hop' then take it out to a game, set the hop and then test it.

 

Often resulting in being massively under the power limit once I got my hop set for my chosen weight (either 0.32 or 0.36) and then requiring me to come back home, disassemble the gearbox to install a new spring to suit. (Later on I bought an SVD-S that had a quick change spring I could change without taking the gun apart. So much easier!)

 

By now, over a year and a half after originally getting it, I ended up with a 650mm 6.02 tightbore (maybe ZCI, I can't remember?) with a smaller window, and just a basic design bucking. (Laylax 60º blue) That seemed to give a reliable, fairly consistent performance that I just wasn't getting with any of the fancier buckings.

 

Gearbox wise, I got one of the FPS nozzles with o-rings, some piston o rings from @ak2m4 and a guarder M110 that ended up giving me around 1.64J once hop was set.

I tried the BLS BB’s, same output roughly with random spikes still. I’ve got the original barrel fitted as I bought a maple leaf inner barrel (510mm as I thought the standard was overkill) but was getting bb’s striking the muzzle on exit. I may try your suggested hop combo though!

 

I love the SVD and would like to get it up to a respectable spec to be honest, but still think the RS may end up being the better choice all around 🤔

[Hudson]

This is how it currently is.

Not used it in a while though, I have to say.

 

I find DMRs to be kind of awkward, I'm likely to use either a bolt action or a normal AEG more often, I should remedy this in the new year!

[Austeyr]

11 minutes ago, Hudson said:

I find DMRs to be kind of awkward

I primarily LMG, but the SVD is just beautiful! (As is yours!) I did quite enjoy the more laid back approach to being a DMR though

[Austeyr]

Gone back to trying to fix this headache. I fitted a Cyma nozzle and cylinder head, fps is all over the place still (114 - 492 on a .28g BB)

 

I noticed the nozzle seems to do this strange “bounce”

[strykerles]

I'd say that's definitely something screwy in the gearbox. I've built 2 cyma svds in the last week and neither do that bounce 

[Austeyr]

16 minutes ago, strykerles said:

I'd say that's definitely something screwy in the gearbox. I've built 2 cyma svds in the last week and neither do that bounce 

Wanna do a third? 😂

 

I think I found the cause, the nozzle spring is a bit worn and not sitting straight 

[Hudson]

17 minutes ago, strykerles said:

I'd say that's definitely something screwy in the gearbox. I've built 2 cyma svds in the last week and neither do that bounce 

 

 

Do you have a preferred bucking you use?

 

I've tested a few and had inconsistent results; mostly maple leafs before settling with a basic style laylax 60⁰

[strykerles]

Atm I have a ML 70 with an omega nub and 6.02 tb barrel in the wooden one (mine)

 

The other I've built to sell and it has a ML 60 and omega nub on a standard barrel. Currently doing around 280fps on a .43

[Austeyr]

41 minutes ago, Hudson said:

 

 

Do you have a preferred bucking you use?

 

I've tested a few and had inconsistent results; mostly maple leafs before settling with a basic style laylax 60⁰

I went back to the original black bucking and nub as thought the ML may have been the issue. 

[Austeyr]

After messing about more and taking the gearbox apart a million times, the issue still persists. FPS is all over the board. I've checked the hop unit a million times too... Very lost. 

 

I thought perhaps air seal issue, but I don't think an air seal issue would cause 200 - 300 fps variations?!

[Austeyr]

I think I found the fault. My piston is cracked near enough all the way down… And they’re out of stock 😭

[Austeyr]

Well, I have finally found the cause. After putting the stock piston back into my replica it was still all over the place. Fast forward a few days and I buy a lovely replacement from @strykerles
 

For shits and giggles I figured I’d try the hop up from his (Which was consistent with nearly every shot) and my old wooden furnitured SVD was also consistent! Huzzah!

[ButcherBill]

Just as well you noticed the cracked piston before you threw it out.

[Austeyr]

2 hours ago, ButcherBill said:

Just as well you noticed the cracked piston before you threw it out.

Wood ones being retired anyway, stocks getting really wobbly due to poor securing points. Only used it once 😂 it’ll be a place holder until I can afford a real sword or LCT