LATEST UPDATE:
THE PROBLEM HAS RETURNED - over the past 2 months despite all of the changes detailed below. The changes did resolve the issue for a few months, but it did not last. I've reached out to FX regarding a possible redesigned valve rod & plenum nose o-ring combination that members have mentioned may be available.
ORIGINAL POST:
My M3 performed flawlessly for the first couple months after I purchased it. I had it tuned to 1/2” groups at 50 yards with the 2nd reg at 150bar and velocity at 990fps +/- 3fps. Every time I took it out it was dialed in. But I did noticed that whenever I took it out after it had been sitting for a day or two that the first 5-6 shots were slow. They would be something like this: 930, 950, 960, 970, 980, 990. I figured I could deal with this. Before hunting I would dry fire the gun around 6 times.
I took a break from shooting during the past 2 months. I did not make any changes to the gun during this time. It just sat in its case. I took it out to shoot. I checked to make sure that the reg pressure, valve and hammer were all set to where they were they had been when the gun was dialed in at 990fps. The first shot was 850fps! Over the course of 20 SHOTS the velocity eventually climbed to 990. I thought maybe things just needed to get warmed up after being unused for so long.
But then I tested a shorter time interval between shots. I took 10 a minute break, then the took a shot. 960! After just 10 minutes a drop of 30fps! Then over the course of 5 shots it climbed back to 990. This is not how it behaved before. If I had taken a 10 minute break 2 months ago, there would be no change in velocity.
The next day I took the gun out again and the first shot was 930fps. 15 more shots and it’s back up to 990.
Any ideas guys?
UPDATE: PROBLEM SOLVED
SOLUTION(S):
I did several things, all addressing the issue of friction between the valve rod & plenum nose o-ring. I made all of these changes at the same time because I didn’t want to disassemble the rifle, make a change, reassemble, let the gun sit overnight, test, then repeat the process for each change. As a result I’m not sure which change had the most significant effect on resolving the issue, but all of these changes could only help resolve this problem in the present and prevent it from happening again as quickly in the future.
For reference here’s a link to the FX Impact M3 schematic:
https://fxairguns.com/wp-content/uploads/2021/04/F10370-Impact-M3.pdf
I disassembled the gun down to the valve rod and on the 2 inch section that slides in and out of the Valvetube End (part 10338) there was a grippy grey residue. This was from the o-ring (part 19768, O-ring 2.84x2.62 PUR90) in the valvetube wearing away onto the valve rod, perhaps due to just normal use, not enough lube being applied at the factory, o-ring shrinking overtime - likely multiple reasons.
I did several things to reduce valve rod friction. I did more than necessary. Most people experiencing valve rod friction could likely fix it by cleaning and lightly lubing the valve rod with silicone oil.
What I did is excessive, but in the long term may require less frequent maintenance:
As described in the original post, before these changes the first 20 shots would be slow: starting at 100 fps slow and gradually increasing with each shot. And if the gun sat for just 10 minutes the fps would drop by 30 fps.
THE PROBLEM HAS RETURNED - over the past 2 months despite all of the changes detailed below. The changes did resolve the issue for a few months, but it did not last. I've reached out to FX regarding a possible redesigned valve rod & plenum nose o-ring combination that members have mentioned may be available.
ORIGINAL POST:
My M3 performed flawlessly for the first couple months after I purchased it. I had it tuned to 1/2” groups at 50 yards with the 2nd reg at 150bar and velocity at 990fps +/- 3fps. Every time I took it out it was dialed in. But I did noticed that whenever I took it out after it had been sitting for a day or two that the first 5-6 shots were slow. They would be something like this: 930, 950, 960, 970, 980, 990. I figured I could deal with this. Before hunting I would dry fire the gun around 6 times.
I took a break from shooting during the past 2 months. I did not make any changes to the gun during this time. It just sat in its case. I took it out to shoot. I checked to make sure that the reg pressure, valve and hammer were all set to where they were they had been when the gun was dialed in at 990fps. The first shot was 850fps! Over the course of 20 SHOTS the velocity eventually climbed to 990. I thought maybe things just needed to get warmed up after being unused for so long.
But then I tested a shorter time interval between shots. I took 10 a minute break, then the took a shot. 960! After just 10 minutes a drop of 30fps! Then over the course of 5 shots it climbed back to 990. This is not how it behaved before. If I had taken a 10 minute break 2 months ago, there would be no change in velocity.
The next day I took the gun out again and the first shot was 930fps. 15 more shots and it’s back up to 990.
Any ideas guys?
UPDATE: PROBLEM SOLVED
SOLUTION(S):
I did several things, all addressing the issue of friction between the valve rod & plenum nose o-ring. I made all of these changes at the same time because I didn’t want to disassemble the rifle, make a change, reassemble, let the gun sit overnight, test, then repeat the process for each change. As a result I’m not sure which change had the most significant effect on resolving the issue, but all of these changes could only help resolve this problem in the present and prevent it from happening again as quickly in the future.
For reference here’s a link to the FX Impact M3 schematic:
https://fxairguns.com/wp-content/uploads/2021/04/F10370-Impact-M3.pdf
I disassembled the gun down to the valve rod and on the 2 inch section that slides in and out of the Valvetube End (part 10338) there was a grippy grey residue. This was from the o-ring (part 19768, O-ring 2.84x2.62 PUR90) in the valvetube wearing away onto the valve rod, perhaps due to just normal use, not enough lube being applied at the factory, o-ring shrinking overtime - likely multiple reasons.
I did several things to reduce valve rod friction. I did more than necessary. Most people experiencing valve rod friction could likely fix it by cleaning and lightly lubing the valve rod with silicone oil.
What I did is excessive, but in the long term may require less frequent maintenance:
- CLEANED the valve rod with rubbing alcohol
- POLISHED the valve rod using a polishing compound and microfiber cloth to polish the valve rod until it had a shinny mirror-like finish. Took about 10 minutes. I used Flitz Multi-Purpose Polish and Cleaner, available on Amazon: https://www.amazon.com/gp/product/B000MUSOW4
- LUBED the VALVE ROD with silicone oil.
- UPGRADED THE VALVETUBE so that it has a PEEK SPACER/BUSHING inside the valvetube next to the o-ring (part 19768). The valvetube that comes stock on the M3 doesn’t have a threaded end that allows it to be removed so that the two o-rings inside the valvetube can be easily accessed. It was difficult to replace the 2 o-rings inside the valvetube. The stock valvetube also doesn’t have a high strength spacer/bushing to support the sealing function of o-ring 19768. The peek bushing also ensures that there isn’t any metal-on-metal contact between the valve rod and the valvetube. For these reasons I purchased a custom valvetube from AGN member mubhaur which has these features. He explains the rational for his design in the video below.
As described in the original post, before these changes the first 20 shots would be slow: starting at 100 fps slow and gradually increasing with each shot. And if the gun sat for just 10 minutes the fps would drop by 30 fps.
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