Tech Advice Needed - Overspin and Inconsistent, Low FPS

[Ragingrental]

I had recently (well to be honest it was a couple months ago) upgraded my specna arms SA-E07  to improve trigger response  but did not have batteries to try it out.  Well today my batteries finally turned up so I  got to try the gun out and chrono it. Accuracy is just fine but there is still overspin  (so bad that it burst shoots on semi) as well as low, inconsistent FPS. As in 200 fps with .25s and 240 with .20's with a variation of about plus or minus 10 FPS. I have ordered a new spring and hopup bucking (current hopup bucking was torn for some reason) but I am a bit stuck in the process of elimination as I don't have much experience with this.

Parts list 

Prommy purple bucking

PDI 6.03mm AEG barrel

DIY Flat nub

Stock hop unit

All gearbox compression parts are stock

13:1 SHS Gear set

SHS High torque motor

Stock M90 spring

Everything else is stock and  the gun still has the X-ASR mosfet.

I am using 7.4v 25-50c 1300 mah lipos. 

Sorry if I missed out some important details - not exactly sure what parts are relevant when trying to troubleshoot such issues.

 

[Adolf Hamster]

What was the fps before? If the springs intact and you havent messed with short stroking/thrust bearings then it shouldnt be dropping energy.

I'd be looking first at air seal, specifically the hop as thats what you changed, a change in feed lip length could be losing you compression.

Double spinning isnt in itself an issue, but might indicate she's going a tad fast, whats the rof in auto?

Fps drop on auto (but not semi) is a sign of pme if your pushing her too fast. Been a while since i've run an shs ht but if memory serves they're much faster than the term high torque would imply, certainly on 11.1 i can see it pme'ing not so sure on 7.4, this is more ducks territory.

If your aim is trigger response (or more accurately lock time- how long between pulling the trigger and the round going downrange) then you'd be better off looking at mosfets with precocking like the gate warfet, it'll make the semi auto nice and snappy but not require excessive auto rate of fire and cure the overspin at the same time.

 

[Tactical Pith Helmet]

If your aim is trigger response (or more accurately lock time- how long between pulling the trigger and the round going downrange) then you'd be better off looking at mosfets with precocking like the gate warfet, it'll make the semi auto nice and snappy but not require excessive auto rate of fire and cure the overspin at the same time.

100% agree.  I have an SHS ht motor in my 'fet fitted M4 CQB rifle.  Otherwise stock but for barrel/hop unit/rubber/shims.  No 13:1 gears though.  ROF/trigger response is great.  

The 'fet makes ALL the difference.  

Look at airseal first, its a right bugger on some rifles.  Then remember the adage that if something mechanical is wrong, it's probably down the last thing you did to it.  

 

[Ragingrental]

Morning everyone. I  unfortunately can't remember what the gun was shooting before but it was about the 300 mark. As for full auto it is shooting 13 RPS! What? No way that's normal. I need to try a different 7.4v battery. 

I'll try the new hop rubber first and also maybe make sure the nozzle to hop seal is good as I'm pretty confident that the gearbox air seal is totally fine. Might also try the new spring. I don't really want to get a gate warfet because I'm not very keen on throwing more money at this yet but maybe in the future I'd consider it.

 

[Ragingrental]

Just re-chronoed the gun. Got some more up to date figures using a freshly charged 7.4v battery and specna arms 0.2g BBs.

Full auto ROF - 20 RPS

Average of 241 fps in full auto

5 shots fired in semi  auto

1- 240.7 FPS

2- 244 FPS

3- 239.8 FPS

4- 241.8 FPS 

5- 245.2 FPS 

Compared to around 290-300FPS before. My new bucking should be here soon and I believe that the bucking is probably the culprit as I had to sand down the feed lips to make it feed properly and that made them quite rough and torn, most likely ruining the airseal. I'll update you guys then but any input is obviously welcome.

 

[Adolf Hamster]

20rps should be fine then.

I believe that the bucking is probably the culprit as I had to sand down the feed lips to make it feed properly and that made them quite rough and torn

yep that's your problem right there.

you could try a PDI W-hold, it's my go-to bucking for when the maple leaf stuff has feeding issues.

 

[Skara]

The stock compression in specnas is bad, I was getting horrible energy losses going from 0.2s/0.98J to 0.28s/0.6J (without even setting the hop), so I recommend you throw away the nozzle and get one with at least one o-ring.

With the same gears/motor and a slightly beefier battery (7.4v 1450mAh 30c) I was getting PME for days, so depending on your barrel length you may want to short stroke the sector by one or two teeth and replace the spring with a longer one (SHS springs are great).

I say replace the spring because bbs act as barrel plugs, increasing back pressure on the piston, the short stock spring didn't have enough oomph to overcome said back pressure and even on a short stroked setup I still had PME .

Also forget about flat hops, they require an unnecessary amount of work (they need a super thick tensioner to make up for the lack of contact patch) and you can just get better results with a W-Hold or a maple leaf rubber.

Moreover a flat hop stretches the rubber a lot more than other setups, which leads to a higher chance of tearing the rubber apart if the barrel window isn't 100% smooth.

It's a thing of the past and it should stay there.

 

[Lozart]

The stock compression in specnas is bad,

Let's not tar them all with the same brush, it's the ones with the Orion gearbox that sucked donkey balls.

 

[Skara]

Let's not tar them all with the same brush, it's the ones with the Orion gearbox that sucked donkey balls.

My Cores don't have the Onion gearbox, yet the compression was almost Ares levels of bad out of the box, even after copious amounts of lubing and o-ring stretching

Technically one does not have to replace the whole compression set, at worst it's a matter of replacing the piston head o-ring and swapping the nozzle with a better one.

On mine I went the extra length and replaced both nozzles and piston heads with FPS parts, mainly for testing as I heard great things about them but never got the chance to try them out. I left the stock cylinder heads as they sealed perfectly against the cylinders and being polymer they help with shock absorption (the piston heads and nozzles are polymer too btw)

 

[Lozart]

My Cores don't have the Onion gearbox, yet the compression was almost Ares levels of bad out of the box, even after copious amounts of lubing and o-ring stretching

Technically one does not have to replace the whole compression set, at worst it's a matter of replacing the piston head o-ring and swapping the nozzle with a better one.

On mine I went the extra length and replaced both nozzles and piston heads with FPS parts, mainly for testing as I heard great things about them but never got the chance to try them out. I left the stock cylinder heads as they sealed perfectly against the cylinders and being polymer they help with shock absorption (the piston heads and nozzles are polymer too btw)

My One is 5 years old now and it's still running the original compression parts.

 

[Ragingrental]

The stock compression in specnas is bad, I was getting horrible energy losses going from 0.2s/0.98J to 0.28s/0.6J (without even setting the hop), so I recommend you throw away the nozzle and get one with at least one o-ring.

With the same gears/motor and a slightly beefier battery (7.4v 1450mAh 30c) I was getting PME for days, so depending on your barrel length you may want to short stroke the sector by one or two teeth and replace the spring with a longer one (SHS springs are great).

I say replace the spring because bbs act as barrel plugs, increasing back pressure on the piston, the short stock spring didn't have enough oomph to overcome said back pressure and even on a short stroked setup I still had PME .

Also forget about flat hops, they require an unnecessary amount of work (they need a super thick tensioner to make up for the lack of contact patch) and you can just get better results with a W-Hold or a maple leaf rubber.

Moreover a flat hop stretches the rubber a lot more than other setups, which leads to a higher chance of tearing the rubber apart if the barrel window isn't 100% smooth.

It's a thing of the past and it should stay there.

Would a maple leaf or PDI W hold work with a PDI barrel? I ordered a G&G green for now just to be able

to go out and play. I do have a lonex M100 spring on the way as well.

So if I get a stronger spring and possibly short stroke a  few teeth there should be less PME? Could I maybe just swap the gear set?

 

[Skara]

Would a maple leaf or PDI W hold work with a PDI barrel?

Yes

So if I get a stronger spring and possibly short stroke a  few teeth there should be less PME? Could I maybe just swap the gear set?

Why go through the hassle of spending time with a new set of gears when you can literally hack the first two teeth off the pickup side of the sector and be done with it?

 

[Ragingrental]

Yes

Why go through the hassle of spending time with a new set of gears when you can literally hack the first two teeth off the pickup side of the sector and be done with it?

Yeah I guess you are right.  Is there any particular nozzles that you would recommend other  than the FPS ones you mentioned?

Also I am slightly worried about the FPS being a bit low with SS + a M100 spring. But I guess it surely won't  be as bad as it is right now. I have heard SS'ing removes about -15fps per tooth? I wouldn't mind picking up a slightly stronger spring in the future, I just want to get the gun skirmish ready by this weekend.

 

[Skara]

It depends on the available air volume and barrel length.

To give some context:

My C08 has a 4/5 cylinder, 247x6.02 barrel and with just one tooth removed and a SHS m90 it clocks in at 0.95J

My C02 has the stock 3/4 cylinder, 260x6.02 barrel, two teeth removed and with a SHS m90 it does 0.99J, on the verge of being hot for my country's law.

Apart from the cylinder, mosfet (irrelevant), barrel length and hop rubber (both Maple Leaf, MR and Super Macaron respectively) they have the same exact internals (SHS HT and SHS 13:1 gears).

Mind you, I didn't do any math when removing teeth, I just went by feeling but kept barrel length in mind when doing the C02 (I did the c08 first). With an M100 it clocked at 1.2J. also PDI barrels aren't as tight as ZCIs (which is a good thing, wider over tighter every day)

I recommend getting a chrono and doing things one step at a time.

I know dismantling and putting together the gun multiple times is tedious but it's the only way to ensure you don't overdo something.

As far as nozzles go, SHS polymer ones are good, I have one in my PDW and it gives a nice 100% seal, I did remove the little bridge on top to get maximum air flow.

Don't go for a metal nozzle though, they tend to shred hop rubbers and do all sorts of nasty shit if not 101% aligned. Also make sure to get the right length, too long and it won't feed, too short and it won't seal against the hop rubber. Measure the stock one with a caliper and buy accordingly.

 

[Ragingrental]

It depends on the available air volume and barrel length.

To give some context:

My C08 has a 4/5 cylinder, 247x6.02 barrel and with just one tooth removed and a SHS m90 it clocks in at 0.95J

My C02 has the stock 3/4 cylinder, 260x6.02 barrel, two teeth removed and with a SHS m90 it does 0.99J, on the verge of being hot for my country's law.

Apart from the cylinder, mosfet (irrelevant), barrel length and hop rubber (both Maple Leaf, MR and Super Macaron respectively) they have the same exact internals (SHS HT and SHS 13:1 gears).

I recommend getting a chrono and doing things one step at a time.

I know dismantling and putting together the gun multiple times is tedious but it's the only way to ensure you don't overdo something.

As far as nozzles go, SHS polymer ones are good, I have one in my PDW and it gives a nice 100% seal, I did remove the little bridge on top to get maximum air flow.

Don't go for a metal nozzle though, they tend to shred hop rubbers and do all sorts of nasty shit if not 101% aligned. Also make sure to get the right length, too long and it won't feed, too short and it won't seal against the hop rubber. Measure the stock one with a caliper and buy accordingly.

I do actually own a chronograph already. I think I did mess up a bit upgrading everything at once - it  made it a bit harder to play the game of elimination. I'll work on the PME and overspin first as that's what will damage the gun over time and it just seems like a logical step. Ensuring good air seal can be secondary.

My edge has a very similar barrel to your  C02, but I am not so sure about the cylinder. I guess I'll just see when I  install the new parts, but your post has convinced me it surely won't be too bad.

 

[Skara]

Almost forgot.

Don't push for maximum fps/Joules, it's a thin line between having a great day and not being able to play because the site chrono is messed up and reads higher energy levels.

There is sod all difference between 1 and 1.1 Joules, maybe two metres of extra range on a good day.

On the overspin issue, short stroking + a stiffer spring will help, but honestly I wouldn't worry too much about it, in a certain way it avoids lock ups

Which imo are worse than a bit of automatic precocking as I'd rather have a weird cycle timing than pulling the trigger to see no BB leaving the barrel

 

[Ragingrental]

So PME and overspin aren't connected in a way?  I was under the impression they were but it's starting to make a little more sense now. Overspin is just a little bit of precocking rather than something that will shred your gears over time.  I guess "premature engagement" is pretty self explanatory. 

So when you get overspin it's simply that the drivechain has too much momentum to be stopped? So it just keeps going and possibly double fires or whatnot?

 

[Adolf Hamster]

My One is 5 years old now and it's still running the original compression parts.

isn't that just the thing with the specna's though, they started off pretty damn good then went downhill?

Would a maple leaf or PDI W hold work with a PDI barrel? I ordered a G&G green for now just to be able

yep they both work. i normally go for the maple leaf macaron+omega nub, however some guns can have feeding issues with the slightly longer feedlips on the ml stuff so the PDI W would be my go-to for those situations.

g&g green is decent, really depends how heavy the ammo you wanna use is.

There is sod all difference between 1 and 1.1 Joules, maybe two metres of extra range on a good day.

yeah but them 2m is exactly where buggers like me are standing

it's true though, all it takes is the site's chrono after years of living in a damp shed and filled with the schrapnel of innumerable tube strikes to be innaccurate and the site will go "nope you're 1fps over get out of here you filthy cheating scum"

being a little bit under is preferable to not being allowed to play.

So PME and overspin aren't connected in a way?  I was under the impression they were but it's starting to make a little more sense now. Overspin is just a little bit of precocking rather than something that will shred your gears over time.  I guess "premature engagement" is pretty self explanatory. 

overspin is the motor/gears having enough momentum to haul your gears around for a second time, PME is the sector spinning so fast it does a full circuit before the piston has finished moving forwards.

overspin isn't strictly a problem, although it is generally a sign that the box is spinning fast, which is the same root cause as PME. overspin can be fixed with mosfets that use active braking.

if you're planning on short stroking and are worried about volume take a look at the port on the cylinder- you need the piston to be going at least that far back.

 

[Ragingrental]

isn't that just the thing with the specna's though, they started off pretty damn good then went downhill?

yep they both work. i normally go for the maple leaf macaron+omega nub, however some guns can have feeding issues with the slightly longer feedlips on the ml stuff so the PDI W would be my go-to for those situations.

g&g green is decent, really depends how heavy the ammo you wanna use is.

yeah but them 2m is exactly where buggers like me are standing

it's true though, all it takes is the site's chrono after years of living in a damp shed and filled with the schrapnel of innumerable tube strikes to be innaccurate and the site will go "nope you're 1fps over get out of here you filthy cheating scum"

being a little bit under is preferable to not being allowed to play.

overspin is the motor/gears having enough momentum to haul your gears around for a second time, PME is the sector spinning so fast it does a full circuit before the piston has finished moving forwards.

overspin isn't strictly a problem, although it is generally a sign that the box is spinning fast, which is the same root cause as PME. overspin can be fixed with mosfets that use active braking.

if you're planning on short stroking and are worried about volume take a look at the port on the cylinder- you need the piston to be going at least that far back.

Aye, I'll do that when I get around to modding. Still struggling to source an appropriately sized nozzle though.

 

[Adolf Hamster]

Aye, I'll do that when I get around to modding. Still struggling to source an appropriately sized nozzle though.

try ak2m4, does a decent range of nozzles.