unusual fps from upgraded spring

[georgehutchison]

I have a full metal d-boys stock m4 bushmaster stlye AEG. The gun is shooting around 340 fps stock. Today I tried a variety of springs to increase the fps as well as some other little hop ups.

First off I tried a modify s120 spring which i have read to produce around 400 fps. Installed it cronied at 320 fps.
Next I tried another 400 fps spring, same thing around 325 fps. I was thinking i must have done something wrong so i went back to the stock spring and nope, back to 350 fps. The modify s120 is at least one inch longer than the stock spring. I dont know if this matters but i really thought it would put it up to 400fps. I tried a new piston, head, sleeve and that made things worse. Now the nozzle protruded slighly into the hop up and stoped ammo from feeding properly. I have had this box apart about 10 times now and I am feeling dumb as to why with upgraded parts it performs worse. Maybe D-boys just have shitty mechboxs that dont like upgrades.

Any suggestions?
Is the mechbox worth upgrading or should I just build a new one from scratch?
Anyone have a recomendation for a good drop in mech box around 400fps.

Thanks

[m102404]

That's really twisted about the weird FPS'. Chrony's ok? Didn't load 0.25's by accident?

Is it a ported cylinder? If it's a full cylinder, the bb might be gone before it reaches top speed.

Of if you have a really long barrel on it...it might not be long enough without a full cylinder...

Stuff like that.

It's ok if the nozzle protrudes a bit, as long as it retracts a bit more before it goes forward...check that it hasn't jumpped off of the tappet plate. A full disassemble and reassemble might change it a hair. Tighten the stock screw.

Or...do what I do now...buy a G&P complete mechbox and just reshim it...LOL!

[Gunny_McSmith]

bad airseal?

[rustysniper]

hey George....are you maybe putting the upgrade springs in backwards? Modify springs aren't linear so there is a front and back....

[lemegacool]

Originally Posted by rustysniper

hey George....are you maybe putting the upgrade springs in backwards? Modify springs aren't linear so there is a front and back....

the end with the more coils at the rear (seating on the spring guide)

[SHÖCK]

Bad airseal or incorrect cylinder porting. Stronger spring is pushing piston faster but air escaping out cylinder port instead of through nozzle?

[georgehutchison]

thanks guys i think i am going to put it back to stock and keep it as a back up mech box. I am just going to try a couple more things. This problem has frustrated me for 2 days now. thanks for the tips i will try them out and report back.

[georgehutchison]

Originally Posted by SNK

Bad airseal or incorrect cylinder porting. Stronger spring is pushing piston faster but air escaping out cylinder port instead of through nozzle?

by just upgrading the spring can air escape out the cylinder port when firing?
how would you solve this. take a tooth off the piston or block part of the port?

[Corleone]

Friend of mine had the same problem on his VFC HK 416, and couldn't figure out why he had such a low FPS. He had just replaced his stock spring with a new 1J one, but it would only give him about +-280 FPS. I had him do the timings differently (correctly), and he got 330+ You should try and check this. I know some people don't believe in timing of gears, and some supposedly even proved that gears reset themselves, but this is not the case with any of my 3 gearboxes or my friend's one. You see, it's all about when the nozzle is let forward into it's foremost position and seals up with the hop-up rubber sleeve. At the same moment the piston is released and it starts moving air. Well this is what's supposed to happen, and it didn't happen on my friend's v2 gearbox, or my own v6, without proper timing that is!

Also, for or better airseal between hop-up rubber and the nozzle, get an o-ring (of the proper size) and thread it over the inner barrel, all the way to the hop-up unit. It might be a tight fit, so don't use too thick an o-ring! This will help press the hop-up unit more tightly towards the gearbox, hence giving you a better airseal between nozzle and hop-up rubber.

EDIT: Guys, I'm pretty sure he didn't say anything about swapping inner barrel or cylinder, only the spring. This would mean that the porting should be ok. As long as the spring is placed the right way around, it should be ok. That's if it's an irregular pitch spring!

[yuhaoyang]

a dboys spring in my custom G&P mechbox output enough eK to go through the side-bottom of a pop-can. And the dboys nozzle doesn't seal. it makes air gush out the bottom. actually, neither does the piston head. yeah... get a modify cylinder set is all I can recommend really... also the DBoys spring is really really short and very strong, which is how it achieves such high fps. Most springs are about an inch longer.

[Corleone]

Originally Posted by yuhaoyang

a dboys spring in my custom G&P mechbox output enough eK to go through the side-bottom of a pop-can. And the dboys nozzle doesn't seal. it makes air gush out the bottom. actually, neither does the piston head. yeah... get a modify cylinder set is all I can recommend really... also the DBoys spring is really really short and very strong, which is how it achieves such high fps. Most springs are about an inch longer.

Poor man's chrono is not really to be trusted, but can give you hints about the speed.

The airseal problem isn't due to to nozzle itself, but because the v2 hop-ups rarely provides enough pressure against the gearbox to provide good enough airseal between the nozzle and hop-up rubber sleeve.

The piston head isn't supposed to provide the airseal (between the cylinder and piston head), but the o-ring on it is. Simply getting a better o-ring for the pistonhead, or maybe an x-ring can make a huge difference.

[yuhaoyang]

the pop-can thing is accurate to about +/- 30-40 fps, either way, it's much higher than what he's getting. Also, a bad air nozzle will have a big effect because of the amount of air escaping the bottom of the nozzle. An o-ring nozzle should cure the problem. The nozzle opening also is responsible for making a seal around the BB to propel it. A hop-up spring that is slightly too weak shouldn't create such a big problem. Because the spring is still there, it pushes the hop-up unit against the mechbox with sufficient force. Well, most springs do anyways. If not, it can always be streched out slightly.

[NoGear]

Originally Posted by yuhaoyang

the pop-can thing is accurate to about +/- 30-40 fps.

uhh the pop can is in accurate. if you put a hole in it its about 300+ fps any other test is basicaly useless. someone did the 400 fps test with a 330 fps gun and completed the test successfuly.

[Styrak]

Originally Posted by Corleone

I had him do the timings differently (correctly), and he got 330+ You should try and check this. I know some people don't believe in timing of gears, and some supposedly even proved that gears reset themselves, but this is not the case with any of my 3 gearboxes or my friend's one. You see, it's all about when the nozzle is let forward into it's foremost position and seals up with the hop-up rubber sleeve. At the same moment the piston is released and it starts moving air. Well this is what's supposed to happen, and it didn't happen on my friend's v2 gearbox, or my own v6, without proper timing that is!

Just stop right now. There is NO such thing as gear timing.

[yuhaoyang]

O.o... for the can test most of mine came in the 30fps range... But then again I always use new pepsi cans with .2g BBs. Although the confirmation chronoing is done at a game probably under different conditions.
gear timing doesn't exist. Think about it, the tab to pull back the tappet is attached to a specific spot on the sector gear. so when the sector gear turns a full cycle, so does that tab, it's completely irrelevant to the position of the other 2 gears.