Hopup Compression Loss

[MultipleParadox]

Hi guys
I finally got the motivation to reopen my gearbox for my KA M4 that had FPS issues since the beginning of times.

Long story short, I have real bad FPS out of my KA, even after having upgraded "almost everything" I could.

Current Setup:

Stock Gearbox shell (King Arms 7mm)
Stock Tappet Plate
Stock Brass Cylinder, unported
Stock King Arms 1 piece metal hopup unit

SHS Brass Nozzle with 1 oRing
Mad Bull Ultimate Bearing Spring Guide - V2
ARS MASK 3G Force DUO for V2 Gearbox (Cylinder Head and Piston Head)
Modify Stainless Steel Shims
StormTech MAX Impact Polycarbonate Piston
Prometheus 300mm 6.03 SS Barrel

Shimmed by gundock, who also changed my gearset for I don't remember the brand but nothing fancy.



Now to the interesting part:

Compression test of piston, cylinder, cylinder head as A1;
When I add the nozzle on the piston head, there's a slight loss between the nozzle and the cyl. head. So I added some teflon tape on the cyl. head. It helped.

Barrel has brand new Guarder Black bucking and nub, teflon taped to barrel
Hopup unit slid on top, teflon taped where it meet the barrel (where the C Clip is)

I put the barrel-hopup assembly on the piston-cylinder-nozzle assembly and tried the compression test again. There's loss, pretty important from the look of it. I added some telfon tape on the outside if the nozzle (I have WAY too much of that stuff ), seemed to have helped a bit, but no miracle here.

From the look, and sound of it, it seems like the hiss is around the hopup arm As if the hopup unit was leaking air. I'm not sure how/if that is possible at all.

I've been thinking of changing the whole hopup unit for maybe a prommy (Neo Strike) or madbull (Ultimate) one... any recommendations in that regard is more than welcome, but I'm not sure this is really worth it.

1. What do you guys think my problem might be, with these infos
2. Should I change the hopup unit and cross my fingers?
3. ????
4. Profit


Thanks

[coach]

shouldn't that cylinder be ported for a 300mm IB?

what spring and what were your FPS readings before fixing and with your current setup?

[MultipleParadox]

Originally Posted by coach

shouldn't that cylinder be ported for a 300mm IB?

what spring and what were your FPS readings before fixing and with your current setup?

Ported Cyl: That's what I thought, but that's the stock cylinder KA decided to put in there ... O_o Original IB length was 315mm. I doubt 15mm would make a diff there but...
Spring: forgot to mention, I changed the spring but don't exactly recall what it is, but I would guess SHS M110

FPS:
That's a hard question, as I always had issues... Factory statement and witnesses from other KA buyers: 380fps
What I got when I first Chronoed it at a game: 365fps
Second Game: (from memory) about 330fps
Third Game: maybe 315fps
Fourth Game: less than 300fps
(you understand I don't have access to a chrono at home)

That's when I bought the listed internals, along with an ASCU2 unit and gave that to a gundoc (Ninja_En_Short on ASC) to install, shim, etc all this.
He told me one of my gear (don't recall the name, the flat bottom one I think) was slightly crooked, barely noticeable, but it was affecting performance. So he changed the gearset.

That was last season. This season I played a single game , and it was reading something under 300fps still. So yeah I think I got THE lemon of the batch... :'(

Once I'll have put enough money in it I guess it should be fine and all, just need to pinpoint the "real" problem... :/

[Stealth]

After re-assembling the mechbox, and the nozzle is in its most forward position, place the complete hop-up unit with barrel against the mechbox.

Shine a light through the feed hole

Peep down the barrel from the business end, do you see ANY light? If you do, then you have a bad nozzle to hop rubber pairing.

[venture]

Originally Posted by Stealth

After re-assembling the mechbox, and the nozzle is in its most forward position, place the complete hop-up unit with barrel against the mechbox.

Shine a light through the feed hole

Peep down the barrel from the business end, do you see ANY light? If you do, then you have a bad nozzle to hop rubber pairing.

Nice. Just added that to my little bag of tricks. Thanks, Stealth.

[MultipleParadox]

Will try this out, thanks

[CDN_Stalker]

Regardless of what KA put in there, you need a ported cylinder in there for that barrel length period, breaking point where a full cylinder can work is about over 400mm I recall (somehow 470mm sits in my mind). With that length barrel and a full cylinder, you are already 30fps lower than a ported one would give you.

Not saying it's your issue, just saying it's an issue to deal with. Maybe it was put in to get the fps down cheaply?

[MultipleParadox]

Point taken - Wil get a new cylinder then

[audi_bhoy]

Make sure the hop-up unit also have the little spring that pushes the unit against the gearbox, otherwise it may cause the nozzle not to reach the rubber and it wouldn't seal perfectly.

You should also make sure that the SHS nozzle has the same lenght has the original nozzle. A shorter nozzle would cause the same thing as the unit not seating againts the gearbox properly.

[MultipleParadox]

I see
The spring is there but doesn't look like it's doing it's job well

Thanks for all the infos

[audi_bhoy]

One more thing, your FPS loss might be due to a compressed spring for a long time. If you don't shoot the gun in semi after each use, the piston spring might be compressed in a half-cycle, and this is causing the spring to lose power over a long time.

[ThunderCactus]

Originally Posted by audi_bhoy

One more thing, your FPS loss might be due to a compressed spring for a long time. If you don't shoot the gun in semi after each use, the piston spring might be compressed in a half-cycle, and this is causing the spring to lose power over a long time.

That's a misconception, I've left a spring fully compressed for 7 months and it only lost 8fps

[audi_bhoy]

Well I did some testing with a couple spare springs and I did see a noticeable decrease in FPS. There's more than just the compression factor in this equation. The spring's material can actually change the results, some alloys are not affected by long term compression, while it does alter other alloys.

The spring in this discussion might just have a bad alloy that doesn't keep his strenght simply by behind installed in the gearbox.

[Stealth]

Originally Posted by audi_bhoy

Make sure the hop-up unit also have the little spring that pushes the unit against the gearbox, otherwise it may cause the nozzle not to reach the rubber and it wouldn't seal perfectly.

You should also make sure that the SHS nozzle has the same lenght has the original nozzle. A shorter nozzle would cause the same thing as the unit not seating againts the gearbox properly.

That tiny spring at the top of the hop unit isn't very effective. You're better off with using some o-rings on the inner barrel.

Hell, even G&P ships their guns with a spring on the inner barrel these days.

[CDN_Stalker]

Originally Posted by ThunderCactus

That's a misconception, I've left a spring fully compressed for 7 months and it only lost 8fps

Depends on the steel used and the way it was manufactured. I have a Modify 90 in my MP5A5 that shot 330fps, and a few months later it was down to 300fps. Some spring steels are better than others for long term compression.