Airleak or Spring lemon?
 

Alright so I ran into this issue today at the game.

I have a JG M4 S-System. I did all the following upgrades myself, but I believe I did fine.

Modify Spring Guide
Modify Cylinder
MASK Cylinder head
Modify Piston head (Non-MASK)
Modify Piston (white)
Modify Nozzle with o-ring
Modify Cut-off lever
Modify S90

I also changed the hopup for a Madbull one piece hopup, with Madbull blue bucking, and SCS.

Now, at the field today, I wouldn't shoot above 200iiish FPS, with .20.
Now, I've had trouble with the SCS, I think it's not properly placed as it doesn't even affect the bucking. But, I don't think it's the source of the problem.

I believe it's either an Air leak, or the S90 spring that's just busted.

I did a compression test with the Nozzle, Cylinder head, Cylinder, and piston set, and it was working perfectly.

Also, the one piece hopup was modded to fit, but I don't see anything wrong with it :/

Thank you if someone can help me here, otherwize, I'll simply go see a local gun doc.

how does the nozzle, on its most forward position seal with the hopup rubber?

Any clue on how I can check that? The hopup moves back to meet the gearbox.

As for the nozzle. I don't know how to verify ^^

mmm.... try opening the gearbox, remove the springs, then push the nozzle inside the hopup unit, block the bb feed whole, then blow air in the barrel, see it you can hear the air leaking.....

PS: yes it sounds wrong, but try it...lol

Alright, I'll see, I'll try tomorrow morning, I'm exhausted from that game haha.

Thank you very much!

Try different hop up unit and nozzle. It might be the problem.

as easy as it's to blame a bad spring, it's rare. it can happen but very rare.
focus on finding the leak.

question, are you using the JG shell? I'm wondering if there are some tolerance issues between the modify parts and the JG shell.

I'd suggest getting a new shell but then you might have fitment problems in the JG body. lol

Yes I'm using the stock shell. It seems fine, but it's my only my first one haha.

Gunny, for your test, you do it outside the GB, with the nozzle in the hopup?

yes, seems fine but it isn't the same as a TM, Modify or Systema box.

IMHO, JG boxes are crap. I have a complete drop in that I 'tried' using in my CA receiver. it seemed fine but the fitment was only slightly off and there was an airleak between the nozzle and hopup.

Now I realize that you're using the JG shell in a JG receiver but I believe the Modify parts are the problem even though they are good parts. good parts in a poor quality box doesn't make up for it.

did you notice if the JG nozzle is the same length as the modify one?

How proficient are you at gun work.

2 very common problems.

1. did you install the spring correctly? compressed coils at the spring guide.
2. Did you accidently reverse the cylinder orientation (only relevant with a ported cylinder)

Did you do the compression test?

It's quite likely that your leak exists between the hopup and the gearbox. Try using your old hopup unit first before ripping your box apart again.

Yes both nozzles are the same lenght. I'll look into a new gearbox shell then.

I can't use the old hopup since I modded the plastic receiver. And it's kinda of a one-way mod

I'll try Gunny's test tomorrow morning

just be forewarned that a new shell might not solve the problem either. I don't know what the fitment would be like in the JG body either.

check that hopup unit again. it should move freely without the mechbox in place and when you press it forwards, the spring should push it back.

do you notice any wear marks on the nozzle?

Did you lube the inside of the cylinder and the piston head o-ring?

Yeah I'm aware of that, I'm just going to open it up and check against the hopup

Nozzle is brand new, and still look brand new when I split the receiver. I'll verify when I open up the GB later

Yes, I used G&P's oil kit. I stretched the o-ring too.

Alright, sorry for the double post.


Alright so I had an issue with my SCS not affecting the bucking correctly. It's fixed.

I did Gunny's test, but remove the nozzle, putting it in the hopup. Blocking the nozzle and the BB entry point with my fingers, and blowing air from the other side of the tightbore, no air leak at all.

After, I placed only the tappet plate and the nozzle in the gun, placed the hopup against it, and then, did the same thing as above. Still no air leaks.

I don't get what's wrong :(

Is the spring correct. With the most coils at the spring guide side? I never saw a true answer to that question. You only said the original spring was linear.

Yes, I meant, on the S90 that is non-linear, tighter coils are near spring guide. When I put the stock spring, it's linear so it doesn't matter.

But, I think I found the leak. Sometimes, the sort of spacer holding the hopup on the tightbore seems to be leaking a bit. I'll add teflon tape and retry.

I also put tape blocking the window in the hopup, where the tracer unit would go. I don't have one, so I'm just blocking it, just in case it could help somehow.

Alright guys, I don't know really what I did, but now, it's working awesomly, with the S90 and .25s.

One thing I noticed, some kind of dust or like small smoke was coming out of the barrel for the first 5 shots, but I guess it was just dirt or sand in the barrel.

Thank you very much for the help!

ya dust, or oil or grease....

IMO you fixed the leak by teflon taping the hopup rubber and barrel right?

Yes, but I'm not sure if it was really leaking that much from there. Maybe it was just not properly seated with the gearbox.

Anyway, it works ! :D

haha content que ça ait marché... :P

....derien... XD