help > My Hi capa is a gas muncher

[ichimaru gin]

Guys, I upgraded my hi capa with shooters design metal slide and frame. With some advice from the gurus in this forum(Sir illusion, Sir Daiviet and Sir Tyson), I did some filing and sanding to make the slide action smooth and optimal.
These are the parts that I smoothen out

rails
base of blowback housing
frame wall where the back of the magazine touching
Outer barrel
spring guide

I also put enough silicon oil on the rails. Racking the gun was no problem and it was smooth.

I think I did the best I could to optimize this gun's action and I still find it gas munching. I can only shoot 21 Bbs with hi cap 4.3 mag. 25 bbs with hi cap 5.1 mag.

I was thinking maybe the mag valve is not aligning properly with the square hole on the nozzle( I dont know what its called, is it the "gas chamber"?). So I did this test : I removed the silicone oil layer on the square hole, Put silicone oil on the mag valve and install the magazine. And then I removed the magazine, and I found out there is a layer of silicone oil on the square hole. that means, the mag valve was in contact with the hole, right?

Can you guys tell me any other cause that would cause high gas usage?
Would replacing the standard marui blowback unit with shooters design one fix the problem?

Thanks in advance

[Daiviet]

hey again,

I removed the silicone oil layer on the square hole, Put silicone oil on the mag valve and install the magazine. And then I removed the magazine, and I found out there is a layer of silicone oil on the square hole. that means, the mag valve was in contact with the hole, right?

That should be correct, since its touching means that none of the gas is escaping through and its all being pushed into the blowback chamber, which is what you want.

I don't think replacing the upgraded blowback unit will do anything, but it's worth giving it a try. Illusion or Tyson might (probably will) correct me later but my guess is that the valve knocker is holding down the magazine valve about a fraction of a second longer than it needs to, allowing more gas out.

Sorry, but I don't have a real solution at the time being but hopefully this might help you figure something out.

[MadMax]

Try this test with an empty mag and a cocked hammer (mag in the gun).

1. pull trigger and hold the trigger down
2. pull upwards on the slide and pull it back
3. keep applying upwards pull to the slide and push the mag down onto a fill tip on your propane adaptor (difficult, but I'm trying to check something in particular)

If gas flows out of the mag with the slide fully racked and pulled upwards, then your firing pin disconnector tab has not been pushed fully down so it disconnects the the firing pin and lets the mag valve close. The mag valve in your GBB is held open for the first 1/3 of the recoil then the left side tab (gun pointing away) pushes down and lets the mag valve close. Sometimes a new slide lets the blowback assembly ride a bit high so it does not reliably hit the valve disconnector tab so a lot of extra gas blasts out. On the way forward, there's another opportunity to hit to disconnnector so if the issue is sporadic you'll just see inefficient gas usage instead of a more severe total mag dump.

Also check to see if your oring/seal on your nozzle piston is seated correctly.

If the test turns up negative on many attempts (i.e. not even a sporadic issue) then I suspect that your reed valve is not pushing forward and shunting breech gas to blowback properly.

[ichimaru gin]

Sir madmax : I havent totally done what you are suggesting, but here is what I do. When the hammer is at decocked position, I looked through the magwell and there I can see the firing pin is at "pushing the valve" position. And then I tried pulling the slide slowly and when the slide pull was at 1/3 position, click.. the firing pin disengage. SO I guess this is normal, right?

[MadMax]

Sure, that should work, but you do need to pull upwards on the slide while doing your test to push the clearance between the base of the blowback assembly and the disconnector tab as much as possible.

If you can't get the problem to show itself then take a close look at your nozzle body assembly. I'm guessing that the reed valve isn't working properly. Even try removing the spring and counting the shot cycles you get with a reassembled gun. It won't shoot pellets, but it'll indicate if the spring is too stiff or if the reed valve isn't sealing forward properly.

[m102404]

Yes...it is normal.

Now Madmax and Illusion have forgotten more about GBBs than I know all together...so I tend to just sit back and read carefully when they're sharing their expertise...

But I think what MadMax was getting at by lifting the rear of the slide was to check that the left edge of the blow back housing was tripping the release of the valve knocker properly and every time. By lifting the slide, you are essentially raising the whole assembly and making it more probable that the nub that sticks up out of the lower assembly on the left side is popping up properly. (if it doesn't the knocker doesn't get disconnected and stays out).

Personally...I'd start swapping parts to see what works better. Piston heads, recoil springs, etc...

[ichimaru gin]

sir madmax, I tried pulling up the slide while at the same time pulling it to the back and yes, the firing pin disengage at the exact same location. So I guess I have to check the reed valve. Can you tell me which part is the reed valve?

Sir tyson, Yes I already ordered shooters design POM cylinder set because I heard one of my friend who owns a hi capa installed this and he said the gas issue was better. What brand of piston head is good?

[m102404]

The reed valve (aka floating valve, power bulb, etc..) is the little cylindrical valve that sits inside the nozzle.

There are 4 parts to the nozzle:
- nozzle housing
- valve blocker, the piece that traps the valve in the nozzle and is screwed in place by the little tiny screw
- valve
- valve spring

The valve is usually pushed rearwards by spring force and when the gas is released the gas is funneled forward to push out the bb. When it gets to a certain point, the valve opens an pushed gas backwards against the piston head/assembly, forcing the slide backwards (Redwolf has a great write up on how this works on their website).

If the valve spring is stronger, the valve will stay closed/rearward longer...more gas goes forward (higher FPS). If the valve is heavy (i.e. metal vs. plastic) the spring is "too weak" and so the valve opens easily (less FPS, more blow back). So it's a balancing act to get the right mix. I've swapped in valves and springs and had it result in poor gas consumption and performance.

The little screw that holds the valve blocker in the nozzle housing can be screwed in too far and can hold up the valve.

Since the rearward movement of the slide/chamber housing trips the knocker release to retract the knocker and stop gas flow...anything that retards the slide from moving back will consume more gas (the knocker is holding down the main mag valve longer). A sticky slide, a strong hammer mainspring (the mainspring pushes the hammer forward, the hammer pushes against the slide and the slide has to push it out of the way to continue backwards), a tight piston head/nozzle fit, sticky valves and weak main mag valve springs, etc...can all contribute to the gas flowing longer.

Of course, if you mag is "riding low", and the seal between the top mag seal and the nozzle is poor, gas will be venting out that leak instead of doing what it should inside the nozzle. Be sure to test function with more than a single mag.

Fascinating system really...a wonder it works at all and a wonder that it takes the abuse that it does and keeps working.

You're into the parts where there are fewer and fewer "set" answers...you'll have to fiddle and experiment until you get the right combo.

BTW, I use a stock TM Hicapa piston head in one of my pistons, and a white polycarbonate one (might be an "ICE" one) in another. The nine-ball one in my setups was way too tight....resulting in poor gas usage. Also, I swapped the o-ring of one piston head (it was really loose) with one that gave a better seal...that made things acceptable and a little tighter.

Best of luck,

Tys

[ichimaru gin]

Sir madmax, I did your test of removing the valve spring and I counted the cycle. The problem was still persist. I can only cycle 24 times. Thats 3 more cycles more. Any other ideas? I was kinda stressed right now arghhhh.....

[ichimaru gin]

Guys, Since this topic was long forgotten, I just want to share what I have done to my Hi capa to fix the gas problem.

I finally got my shooters design nozzle and PDI piston head. I had them installed and I realized that the combination of PDI piston head and shooters design nozzle make them tighter, thus causing better seal( this setting is even much tighter compared to marui standard piston and nozzle combination). Other things that I realized was that the PDI piston is thicker than marui's original one, so the gas flow that causing the blowback would hit the piston earlier, resulting in earlier valve knocker release.

So I filled up the mag, crossed my finger and the problem was fixed. I was able to shoot 45 BBs using hi cap 5.1 mag. I said "wow, this was quite a relieve" because I was only able to shoot 23 BBs before I changed all those parts.

I hope this info could be useful to someone who mod another hi capa and have the same problem as mine.

Once again, thanks to all gurus in this forum. Ciao