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Post by Tank on Aug 13, 2014 18:48:15 GMT -5
I recently bought a G&P CQB M4 and tore it all apart this week and modified just about everything. This in itself makes tracking down the source of my issue a lot more difficult, but I thought I'd throw my problem out here and see if anyone else has encountered this before.
Before the rest of the mods (or the issue):
This rifle has a 285mm Prometheus 6.03 barrel in it that has been r-hopped. Previous to doing the rest of my mods, I tested the FPS for this gun and it was shooting very consistently between 333 - 335 FPS with the .30 Bioval BBs that I normally use. This is roughly the equivalent of 410 with .20 BBs. Every shot, from the first all the way to the last was at this velocity.
The mods that were done:
I then changed out the gears, from the stock G&P gears, to a SHS 13:1 gear set. I also changed the stock motor that was a G&P M120, to what I now have which is the Matrix Godzilla Super High Torque motor. I also changed out the stock trigger group for an ASCU MOSFET Gen 3. This MOSFET came with a new Cutoff Lever and also a Tappet Delay. I installed the cutoff lever because you have to, and I also installed the Tappet Delay since I noticed that the original gear set had some sort of delay mounted on the sector gear as well. I then corrected the AoE with some rubber washers and also a sorbo pad to the correct spacing when the sorbo is compressed. I changed the piston out for a lonex red piston (which now that I think of it, I forgot to swiss cheese the piston). I lubed everything up using silicone oil in the cylinder and silicone grease on the gears. I checked the compression of the cylinder head and piston head and all checked out fine. The air nozzle was never changed, although I think I forgot to put a little oil on the oring inside it when putting it back together. The tappet plate seems to move freely with everything all closed up.
Issue:
My first shot shoots about 200 FPS with the .30 BBs and then every shot after that climbs by a few FPS until it peaks and then stays consistent at 255 FPS, again with .30 BBs. This is roughly 312 FPS with .20 BBs. This is all with firing semi auto at about one shot per second or two. If I leave it sit for a while, it then starts all over again.
Other things I've noticed:
The air nozzle does not rest fully forward into the hopup, between shots. It appears that the ASCU is stopping it when it is slightly starting to pull back again.
It seems as though I may have put a slight bit too much oil in the cylinder since when I hold my hand right outside the air nozzle and fire the gun, my hand gets very slightly wet after 5 or more rounds. Just very slightly though.
The gun seems to sound great, it's not too noisy from the gearbox and the sorbo dramatically reduced the clack sound from the piston slapping the cylinder head.
Any thoughts guys?
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Post by Stinger on Aug 13, 2014 19:07:03 GMT -5
By putting a sorbo pad in the cylinder, you lower the volume of the cylinder and thus the amount of air you can displace behind the BB, resulting in less muzzle velocity.
My first thought about the "climbing" FPS issue is that I occasionally have the same issue with my primary. When I load in a magazine, the first five shots might be about 80 FPS lower than my normal average, and then it climbs after that back to normal and stays that way. Now if I load a magazine, shoot three shots, and find that they are dropping, I pull the mag out and then immediately load it again. Generally, the problem is fixed by that. My guess is that there is a compression leak between the hop-up and the air nozzle due to misalignment of the hop-up chamber. After a little bit of shooting, or moving around the magazine, it gets back into its normal position. I don't know how to permanently fix it, but that just seems to happen to me every now and then.
The air nozzle shouldn't rest fully forward unless you decompress it fully (this means pulling the anti-reversal latch).
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Post by Tank on Aug 13, 2014 19:19:51 GMT -5
I expected an FPS drop, from the AoE correction, but I didn't expect as much as I got. I can live with it though, if it would just stay consistent.
To be clear though, if I let the gun sit a while without firing it, or removing the mag, it fires slow again and climbs again.
I wasn't sure about the air nozzle position, so I thought I'd mention it. Thanks for the clarification.
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Post by Stinger on Aug 13, 2014 19:34:42 GMT -5
I expected an FPS drop, from the AoE correction, but I didn't expect as much as I got. I can live worth it though, if it would just stay consistent. I wasn't sure, so I mentioned it anyways. I agree that the drop should not be that much, and it is likely due to a compression leak. When you checked the compression, did you have the air nozzle on it? I always check it first without the air nozzle on the cylinder head, and then with it, to pinpoint any leaks. If you physically cannot get the air to leak out whatsoever, you have a perfect seal. Obviously this is optimal, and it is totally achievable with the right parts and fitting. To be clear though, if I let the gun sit a while without firing it, or removing the mag, it fires slow again and climbs again. I once heard someone talking about his bucking having to "warm up" or "stretch out" before it would properly hop the BB. Maybe that is messing with the muzzle velocity? I dunno. I also wonder if your tappet delayer could be messing up the timing of the air nozzle, causing it to have to cycle a few times before it is in place. You could try taking it off and then testing it. As you can see I'm just shooting off random ideas, as this is a bit of a weird issue.
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Post by Tank on Aug 13, 2014 21:08:06 GMT -5
Random ideas is what I'm hoping for, honestly. I agree, it is a very bizzar issue, which is why I felt the need to post it. I've literally found no one talking about this online.
I was going to ask if this was a common Sorbo drawback, but since I didn't see anything about it, I assumed it was something abnormal.
One thought X and I had was that maybe the Sorbo gets compressed some when multiple shots are taken and then it naturally has a larger cylinder volume, therfore it shoots faster. But we both thought it was a real stretch, for that to be the case.
I was questioning the Tappet Delay. Potentially it is opening up the seal with the hop up too soon and losing compression. The only reason I used it was because the original sector gear had some sort of delay on it, so I figured maybe this gun might need it. I'm kind of dreading taking it all apart again, just to take that out, but now I'm curious. However, where I can see this potentially fixing the drastic overall FPS loss, I don't see it fixing the climbing issue. :/
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Post by Stinger on Aug 13, 2014 21:35:23 GMT -5
Well the best I can tell you is this: Don't dread taking it apart. Don't be afraid to tweak, fiddle, or even mess up. It's the only way you'll get a gun that shoots perfect (or close). It's also how you learn. These guns don't work by magic. If something gets messed up, it can always be fixed.
Since you didn't systematically upgrade by installing, then checking...installing, then checking, etc. (a lot of work), you now have to systematically undo what you have done to locate the cause of the problem. Take something out, and test. If it does not improve, reinstall it and then take something else out, and check. Repeat until you figure out what it is. Like I said it's a lot of work but it is worth it in the end when you have a setup that just rips.
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zephurah
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Post by zephurah on Aug 13, 2014 21:52:45 GMT -5
If it fired perfectly before you replaced all of the parts, then there is a easy way to solve this: Put the gun back to the way you got it stock, and start adding parts. Theoretically you should be able to find the part that is causing this issue.
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T6e9a
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Post by T6e9a on Aug 13, 2014 21:53:05 GMT -5
The delayer might be off as mentioned, (something I would check) Possibly shorten the tappet plate spring so that it might return faster, help with timing possibly. (Just some ideas, no solid theory) Use O-rings to shim the hop up to the gearbox for more consistency, instead of the spring. I have head of people sanding the front of the tappet plate to allow it to go forwad just a smidgen more. See how the tappet plate rests against the axle of the sector gear, if the tappet with spring hooked up sorta pushes the gear off axis, you might need to modify. Will prevent from the tappet being pulled back too far at rest.
Just some more ideas to spit out and possibly think about. Personally sounds more of a mechanical issue than something the ASCU might be causing.
And as for the sorbo, it shouldn't reduce the volume that much to reduce the FPS that significantly. But my idea on the 'warm up' or 'climbing' might be that the sorbo expands and after shots, gets matted down, sorta spreads out into the cylinder.
Still a peculiar issue that will be interesting to see the results of how it is resolved.
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Post by Tank on Aug 13, 2014 23:06:26 GMT -5
Well the best I can tell you is this: Don't dread taking it apart. Don't be afraid to tweak, fiddle, or even mess up. It's the only way you'll get a gun that shoots perfect (or close). It's also how you learn. These guns don't work by magic. If something gets messed up, it can always be fixed. To be clear, I'm not afraid to take it apart, I just hate the whole stressful putting the gearbox back together when it all would rather just explode due to all the springs under tension. I'm generally not disinclined to opening mechanical things up and tinkering with them. I just had this thing opened and closed so many times, trying to solve a different issue already, that I'm kind of tired of it, ha. See how the tappet plate rests against the axle of the sector gear, if the tappet with spring hooked up sorta pushes the gear off axis, you might need to modify. Will prevent from the tappet being pulled back too far at rest. You know, thank you for mentioning this. Now that you brought that up, the way the Tappet Delay mounts on the sector gear involves a thin ring of plastic that encircles the axle of the sector gear, this would prevent the air nozzle from going as far forward as it use to. I'll have to have a look at that.
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Post by Kjones on Aug 14, 2014 1:16:23 GMT -5
My guess is a lack of grease in the cylinder ( use grease not oil in the cylinder, oil is too thin and moves too much and gets shot out the barrel and onto your hop-up, this could lower FPS too, clean your hop-up rubber ) and your o-ring is either worn down or not stretched out enough. What I believe to be happening is your first few shots your o-ring does not make a very good seal ( I know you said it does but maybe it really dont) due to lack of grease or worn down but after you fire it a few time the friction and the rapid compression of the air ( it really does heat the air up a few degrees) cause your o-ring to expand against the cylinder walls giving you your FPS back. This is a very common problem in the winter months, your gun sits in the car on the ride to the field and the o-ring and hop-up contract due to cold and give poor fps. If you do a 5 second full auto burst everything warms back up and works again. To fix your issue I would suggest replacing the o-ring and making sure you have a nice thin coat of grease in the cylinder to give good compression and proper lube. I would not expect your sorbo to be a issue because when the gun is just sitting, the piston is always compressing it and they are pretty solid anyways, they don't compress or expand that much.
IF you ever find the solution to this problem please make sure you post what you found to be the issue and how you fixed it. Other people on MIA or people through out the internet may search for it on google. Reading the exact issue you are having but never getting a answer is incredibly annoying.
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inj
New Member
Just a lonely, solo Michigan airsofter
Posts: 57
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Post by inj on Aug 14, 2014 9:01:14 GMT -5
I really don't know what the issue is, but there's a way to test if it is a bucking/nozzle problem. Forgive me if someone's already mentioned this, I just kinda skimmed through this thread.
Let the gun rest for a while, as the velocity seems to go down as it rests, correct?
Then flip the gun over with the mag out and put a small piece of tissue over the hop up chamber feed tube. Put your finger lightly over the barrel (make sure it's not loaded!) and fire the gun. Don't press hard with your finger, as that's only meant to replicate the little bit of resistance a BB gives in the system. Does the tissue paper fly off? If so, you've got a leak there. Fire the gun a few times until the velocity increases, then do the test again. If it doesn't fly as far or not at all, the nozzle and bucking are beginning to seal better as you fire more.
If you can't figure out the problem, I just recommend taking apart, cleaning, and putting back together all the hop up and compression parts. There may be oil getting into the bucking or something like that. Good luck.
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Post by Tank on Aug 14, 2014 10:51:30 GMT -5
That's actually a really cool idea. I was going to use my gopro in high speed mode and then play back the video slower, to see if the air nozzle was pulling back before it fires. This idea would really tell me if it had a leak there though. Clever.
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ctres
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Post by ctres on Aug 15, 2014 8:57:45 GMT -5
I would personally lean towards this being an air nozzle/tappet plate issue as well. Sorbo is definitely not the culprit here since most guns are overvolumed and the bb would have already exited the barrel by the time the piston head reaches that part of the cylinder. First make sure that the tappet plate slides freely in the gearbox when it is closed. Remove any sector clips as they probably won't be needed unless you are running a pretty high rof. Lightly grease or oil your compression components using your preferred lubricant. I prefer superlube or a heavy silicone oil. Check for compression by putting your finger over the nozzle with the air seal components in one half of the gearbox and pushing the piston forward. Make sure to do this with the nozzle in the forward position. Assemble the gearbox and check that the tappet still moves properly and springs forward nice and crisply with all the other components installed. Turn off all special features on that fet such as AB, etc then try firing.
The last G&P I had used an aluminum nozzle with a very flat front. Are they still the same? Maybe try a different one if none of the above suggestions work.
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Post by Tank on Aug 15, 2014 11:27:11 GMT -5
So I tore it back apart last night. I didn't remove the Tappet Delay. I was going back and forth as to whether I wanted to do that. I decided to roll with it in. The nozzles are still metal but it has a decent chamfer on the front and an oring for a seal. I made sure to get an even better compression out of the piston head, it was really decent before, now it doesn't budge at all if you put your finger on the cylinder head nozzle. With the air nozzle in place, it has just a very slight leak. Overall, I would have to say the seal should be decent enough now, definitely better than it was when I got the gun, and it was shooting 410 with .20's. I also cleaned the barrel and hopup as well.
Now for the interesting part. When I tore it down, I noticed shavings from the piston in the gearbox. Looking at the piston, I messed up and cut too many teeth back when I was correcting AoE which then lead to it starting to get tore up. I still had the stock piston and correctly shaved the right amount of teeth off. I guess the moral of that story, don't work on correcting AoE at 3 AM. Ha! I fired the gun last night and it sounded much better and seemed to be putting out a bit more air. I think with too many teeth shaved off before, it was not fully pulling the piston back, which of course would have lead to the greater reduction of FPS. I really don't have any time to test out the FPS today, so I guess I'll see how it is doing when I get to Grayling tonight. *crossing fingers* I do have my trusty old backup, so I'll be fine if it falls apart on me.
I still can't explain the ramping in FPS, but we will see if that is magically fixed now.
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inj
New Member
Just a lonely, solo Michigan airsofter
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Post by inj on Aug 15, 2014 11:44:36 GMT -5
I wouldn't count on it magically fixing itself. And if it does, I'd still try to go through the old and new parts and try to figure out what was wrong. Issues that fix themselves are always great until they come back a couple weeks later...
Either that or you could go by the "don't fix it if it's not broken" rule and just see what happens. Depends on what kind of person you are.
Either way though, good luck. Keep us updated on how it works.
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