Tuning M3 Suddenly Dumping Air- Help, please!

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jmarkreed

Member
Oct 3, 2020
50
24
MS, United States
FX Impact M3 Sniper, 700mm barrel, .22 caliber



I have throughly enjoyed my M3 and have had absolutely no problems until now. I treat it in the most pristine manner- bench shooting only. Two days ago, I charged the tank to 240 bar (I never go higher than that), and started a shooting session. Within a 4 shots the Sekhmet gauge was reading 80 bar, but I heard nothing that sounded unusual- no hiss or unusual shot sound. I recharged back to 240 bar, and the same thing happened within three shots. Again, no unusual air rush or hissing sounds. I then refilled back to 210 bar and let it sit overnight. The next day, yesterday, the gauge read 106 bar. Today, the gauge continued to read 106 bar, and I filled it to 185 bar and performed a leak test with soapy water at every possible point of air escape. Using a magnifying glass and long observation, I found no evidence of an obvious leak. The gauge remained at 185 bar. I then shot 2 shots. After the first shot, the gauge read 184 bar, but after the second it rapidly reduced to 100 bar. I could not hear any hiss or air flow, and the shots sounded otherwise normal.



(Each time the bottle pressure dropped 100 bar, the 1st regulator gauge was in agreement. I always keep the 1st regulator at 150 bar and the second at 105 bar.)



I then removed the bottle from the gun and heard a complete decompression through the opening of the bottle connector coupler. The gun was completely degassed. This has never happened before when I removed the bottle.



Help, please! This is obviously something that needs fixing, and I really love shooting this amazing gun. I'm positive someone here has a complete explanation of what is happening and how to fix it. 



Thank you very very much in advance!
 
Your gun will always completely degas when you remove the bottle if your pressure is below the second regs set point. Or at least super close. If I have to disassemble my guns, I don’t like constantly playing with a reg, it’s just not good for it. So I shoot the gun down to my second regs set point then remove the bottle. The whole gun degasses and I don’t have to touch either reg after I reassemble the gun. FX Impact tip for the day. If you are rapidly bleeding off 85b, it has to be detectable unless your blowing it out your barrel. Do the same test and get your ear up to the muzzle as fast as you can after firing a shot. Good luck.
 
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@Vetmx thank you for your suggestion. It's definitely the second shot that drops the tank to 100 bar, and I was able to tell that it is not coming from the barrel. It's so strange. I have great hearing (recently tested), but I absolutely cannot hear where all this air is leaking from. It happens in around 4 seconds. With that volume of air, I would think that I could hear something. This is very frustrating. 
 
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What’s interesting is when I would degas one of my MKll’s that had a Huma first reg, I would have to loosen my gauge to degas the gun. Even at a full hiss, it took a while to bleed off 100b. If your second shot blows off 100b, your gun would sound like a cannon. This is very interesting. But leave it to an Impact. The gun that can make just about anybody scratch their head.
 
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@Bigragu, I essentially ended up spraying the whole gun especially under the gauges, inlet port, valve adjuster area, bottle connection area, breech, plenum, and even the bottle itself. I took a flashlight and a magnifying glass and carefully examined each area while filling the gun and after on both sides. I didn't see one new bubble. 

At @NYmike recommendation, I'm going to try to listen at the breech after the second shot. Yes, I agree, it's a good mystery, but I really hate it right now. Thanks for everyone's suggestions. Really appreciated!
 
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1. Listened at the breech after the 2nd shot and could not hear anything. 

2. Changed the Sekhmet gauge out for an FX gauge, and it didn't make any difference.

3. Watched the gauge after the 2nd shot, and it took between 0.5 and 1 second and then it suddenly decompressed to 100 bar, but I still couldn't hear anything. 

4. Took my stethoscope (I'm a physician), and after filling, I could not hear anything.

5. Rechecked key areas with soapy water with no obvious leak

6. The gauge seems to jump around wildly with slow filling making me think that something must be going on with the valve.

7. The key to the mystery has to be the fact that it occurs only after the 2nd shot and that it start to decompress within approximately 1 second after the shot. 

8. I still can't hear anything which makes me question my recent audiogram. 

9. This is academically interesting, but still very frustrating! 
 
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Ok not recommending it, but at this point I'd be tempted to drop the whole gun in a bathtub full of water just to verify that air is actually escaping the gun. To me the key seems to be the lack of any sound.

Are there physical signs that pressure is actually lost beyond what the gauges are telling you? When you refill does it take the normal amount of time? How are you filling it compressor or tank?

The only deduction I can come up with is perhaps the bottle valve is locking up after the fill and you're getting those two shots from the air in the plenum. Either that or the first reg is doing something wonky but I don't know how that would produce these results.
 
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Ok not recommending it, but at this point I'd be tempted to drop the whole gun in a bathtub full of water just to verify that air is actually escaping the gun. To me the key seems to be the lack of any sound.

Are there physical signs that pressure is actually lost beyond what the gauges are telling you? When you refill does it take the normal amount of time? How are you filling it compressor or tank?

The only deduction I can come up with is perhaps the bottle valve is locking up after the fill and you're getting those two shots from the air in the plenum. Either that or the first reg is doing something wonky but I don't know how that would produce these results.
Click to expand...

Same, there's a couple really weird things about this. 100 bar lost in seconds but silently?
 
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Thank you all for your assistance with this. I'm not sure what the bushing is on the valve stem. I've had the same results trying two different bottles- the original FX bottle that it came with as well as the 700 cc bottle that a lot of us have been ordering lately. Same with both with different bottle valves. 

I am refilling from a SCBA bottle with the Edgun Easy Fill system. It does seem strange that when I start to fill it doesn't seem to be doing anything and then suddenly it starts to read a much higher pressure. That's why I changed the Sekhmet gauge back to the FX gauge to make sure I wasn't having a problem with the gauge itself. The FX gauge did the same thing. I know the pressure in the bottle has to be greater than 150 bar since the first regulator gauge reads 150 (where it was set). When it show the big drop after the second shot, it always returns to the 100 bar which is essentially what the second regulator is set. I suppose the lead has to be proximal to the second regulator otherwise the plenum pressure would drop also. Is that true?

Thanks again for all this brain power and experience trying to help solve this mystery. I really appreciate you. 
 
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Ok my second thought was the first reg. They seem to be prone to funny stuff. I've already replaced one on an Mk3 that was acting weird. Creeping up, then taking too long to return to set point after a shot. I just haven't worked out how it could cause the symptoms you describe. But if it were me that's where I'd start blindly replacing suspected parts.
 
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