GTA
General Discussion To Gateway To Airguns => Gamo Gate => : daved December 29, 2006, 05:27:30 PM
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Well, my CFX has decided to act up again. Was shooting some chrony strings the other night, and suddenly the velocity numbers were all over the place. Variations in excess of 150 fps, and an overall drop of more than 100 fps. Since the rifle was tuned less than 6 months ago, and the spring seemed fine, my first suspect was a breech seal. And I was pretty sure it was probably the rear seal on the rotary breech. Talked to Bob, and he was nice enough to mail me some. Tore the gun down, replaced BOTH breech seals, no change. Could it be the piston seal? Had the original Gamo seal that Bob sent back after tuning my gun, went ahead and tried it. Still no improvement. I realize that may be meaningless, but I wasn't looking for improvement as much as just something different.
So now I need ideas. The piston seal that was in there looks fine, even under high magnification. How much pre-load does a main spring have? Mine has about an inch or so, that's not the stock spring, I think Bob used a Tarantula spring. Can a spring go bad and still look okay? BTW, this is the first time I've actually checked out the guts of one of my guns. So please bear with the silly questions. I tried the paper over the breech thing, so I don't think the problem is with the breech seals. I really don't know where to go next, any ideas would be appreciated. Hey, Jeff, wanna trade that new $74 Shadow 1K for a (VERY) tempermental CFX? Thanks guys.
Dave
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Bummer dave,
I wish I had a solution for you, but I own no CFX and have never had one apart. As compared to other springers I've torn into a little over and inch of preload doesn't sound off to me, I could be wrong, just my limited experience says it doesn't sound like the problem. More likely sounds like a leak somewhere, or something just not aligning properly. You say you've tested the breach seals with the napkin over the breach trick, and you replaced (albiet with an old one) the main piston seal. Huhm, just went to Gamo's website and downloaded the manual....looks just like the manual for my S1K except for the loading procedures page. Might be a dumb question, but are you using a decent consistantly sized ammo, most likely so....but just a thought. Sorry Dave, I'm sure someone else will chime in soon.
Dan
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Hey, Dan,
I wish pellets were the answer. JSB Exacts in several head sizes, Superdomes, Beeman FTS's, none of them are working right. Just recently, Superdomes were right at 900 fps with an extreme spread of 20 fps. Now it's doing about 768 and 59. And that's if I throw out one really wild shot. With that one back in, the avg jumps to 780, but the spread goes to 161 fps! Exacts are no better, should be around 860 and less than 10 fps spread, instead I'm getting 100 fps slower and a spread of 66 fps.
The only thing that really makes sense here is that the piston seal has failed. But it looks fine. No nicks that I can see. It looks like it's been sanded for about 1/2 the thickness of the seal, exactly where and what I'd expect to see on a used seal that had been sized to fit my gun. I'm still stumped. But thanks for comiserating!
Dave
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Sorry to hear this Dave,
I would think if it was a spring that it would be consistantly low fps numbers. Just my thoughts. I have three CFX's and have not experienced anything like what you are going through. My CFX Royal .22 I purchased from the UK shoots below average but has less than a 15 fps varialble. THe other two CFX's are good to go with very little variation.
I sure hope you find out what is the problem so you can share this and maybe one day the solution will help another member who is haveing a similar problem...
Best pf Luck
Gene
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...is low in a Shadow, suspect it would be low in a CFX as well. Yank the spring, check the overall length and see if any coils are busted. It can happen with any spring.
Either the Gamo, Macarri, or Crosman springs ought to be on the order of 10.5" free length when used in a Shadow. Again, I suspect the CFX is similar.
If not, let CDT know what kind of free length you've got, he'll be able to tell if that's a problem.
BTW - how fast is is shooting and with what sort of pellet?
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Shadow, CFX, and Hunter all share the same powerplant. I've had the spring out already (TWICE!), but didn't think to measure it. No busted coils, though.
Velocity with Superdomes is 780 avg, 161 fps spread. JSB Exacts are doing 760 avg, 66 fps spread. I was getting right at 900 fps with Superdomes before, and around 875 with Exacts, as I recall. Not sure on the Exacts, but it's close.
Haven't had the chance to do anything today, may go pull the spring after dinner and check it. Later. And thanks.
Dave
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Looks like you're on to something, Vince, free length is just under 9.25". BTW, just to make sure we're on the same page, when you say free length, I'm assuming you mean the length of the spring just laying on the table. In these circumstances, set length would be the same thing. Right? Is spring collapse a common problem? Don't know what he's using now, but I think Bob said he was using one of the Tarantula springs. The specs match up with what I have. Unfortunately, JM's specs don't include free or set length. But it seems reasonable to expect it to be at least as long as a stock spring. Actually, a bad spring is a better solution for me right now, as opposed to some odd sealing problem, since I have a gas ram on order for this gun. Problems I don't mind (MUCH!), it's not having answers that really bugs me! Later, guys.
Dave
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...and both Macarri and Bob say that the "set length" (which is the same as "free length" after the gun's been used) should be 10.5-11 inches.
Springs get shrunk like that from being overstressed. Bob tells me that it comes from excessive dieseling (which has happened to me a number of times), usually from someone putting oil in the compression chamber IN FRONT of the piston (through the transfer port). Or from shooting heavy pellets (>10gr) a fair bit.
While you have it out, can you measure the spring diameter? Do you have a dial caliper of some sort?
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I have a digital caliper, and I measured everything! OD is about .790", ID about .533", wire dia. is .128", looks like 31 coils. Actually, I've come up with a theory about this, but it's specific to the CFX, because I think the cause is the rotary breech. None of the reasons you listed apply in this case, no deisleing, no oil in the compression chamber, and no heavy pellets.
I think the reason for the failures is piston slam due to a design flaw in the rotary breech. That rear o-ring is supposed to seal peak pressure, yet still be soft enough to allow rotation for loading. I think the rotary breech is the reason the Shadow is generally considered more powerful than the CFX, that rear o-ring is almost certainly by-passing some air. And I think it also means the front seal is getting even more abuse than we realized. Not only is there the shearing action of loading, the entire rotary breech has to be getting slammed forward with every shot as well. That's a lot more abuse than a Shadow's breech seal has to put up with!
So here's the challenge for you machinist types. How do you increase the sealing strength of the rotary rear o-ring, while still allowing it to rotate for loading? I beleive the current o-ring is a Buna or Viton, 70 or 75 durometer, 20 x 1 mm. Bob says he's worked on this problem before, harder rings were virtually impossible to install without damage, and were too hard to turn anyway. I have a couple of thoughts, one would be a longer rotary to allow an extra sealing ring, the other is a seperate press fit seal that goes behind the rotary and ahead of the piston. I'll be playing with this, and will post any interesting results, but I'd love to hear any other ideas or suggestions. Later.
Dave
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It doesn't explain why I've had the same problem.
I had a used Macarri spring that was 10" long laying around. I installed it into my 440, and ran maybe a couple hundred pellets through it. Again, no dieselling (at least not enough to notice), no heavy pellets. I had made a tight-fitting delrin rear guide for it and used the standard (all steel) Gamo tophat.
Maybe a couple of hundred rounds later it's down to about 9.25 inches.
I'm at a loss to explain it. Maybe it doesn't like the Gamo tophat? Too heavy or too light? I dunno.
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Well, I did say it was a theory :-)! So how's this for a modification? JM feels that ALL Gamo's have oversize, unchoked barrels, and that because of this, every shot is a dry fire. Which means every shot would have at least some piston slam. Regarding the physics of varying the weight of the top hat, way beyond me, brother! Maybe Rich or Bob have some ideas there. Or you could go to the Yellow and ask Steve in NC, I'm sure he'd have an answer :-)! The odd thing here is that this wasn't a gradual change, it went from good to bad in the space of a shot or two. So I don't know what to think, either.
Here's a thought. If it is a barrel problem, maybe Rich can help out. Take two identical new Gamos, do identical tunes, but put a LW barrel on one. Then shoot both, same pellets and number of shots. Chrony then tear down after every 1000 (?) shots, see what changes when and where. Maybe the forum members would be interested enough to help pick up the tab. What do you think?
Dave
PS, finally checked my email. Yes to pix, price?
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I suspect not - I doubt the choked barrel gives that much braking to the piston, given the fact that it doesn't neck down that much and it happens right at the end of the pellet's travel. I believe that the major "piston braking" effect comes right before the pellet starts moving, when pressure has to overcome the initial push of the pellet into the barrel (to engage the rifling and size the skirt)
Bob says that he usually doesn't see this problem. I wouldn't ask Steve in NC in any case... even if he is technically competent (I honestly don't know), it can be hard to tell good info from his personal BS.
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Well a choked barrel of 17" slowed down a Shadow by almost 80 fps compared to the same rifle with a stock 17 1/2 "barrel, so it does have an affect on the rifle and the compression cycle, and I mean the SAME rifle, now putting on a 15 " barrel on the rifle returned the fps and did not decrease the accuracy of the rifle.
The choke on the Walther barrel is less than .003 if an inch and is only the last 3/4 of an inch of the barrel, this is what sizes the pellet as it leaves the barrel.
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A choked barrel will most certainly slow down a pellets velocity. How much depends on how much the choke as well as any other irregularities (which are also common) in the barrel.
Also, in the case of the CFX, I do see, although not in all of the CFX's, the spring winding up with what I would call "a short set" although how much does vary from gun to gun. If they do set short, it's not too uncommon to see them at between 9-9 1/2 inches. It also happens occasionally in the other Gamo's but no where near as often. And when it happens in the other models, it's usually caused by using incorrect pellet weights. I've seen it happen with both the factory springs as well as the Maccari springs. It may have an influence on velocity but shouldn't influence accuracy at a given distance as long as the velocity is consistant. The early breaking or the "short set" of the springs in the CFX is not the fault of the springs. It also appears that there is not a one single factor that contributes to this phenomenon but perhaps several contributing factors or combination of.
It's funny but some time back I made a statement to the effect that when I tune a Gamo CFX, there will be no doubt about improving it, but you never know what you are going to wind up with. That got pushed in my face by some from another forum but they didn't know what I knew.
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Hey I'm a newbie and thanks to this forum am picking up as much knowledge as I can. I have a silly question about these results that Dave is seeing with the CFX. Is there any method of checking a crony for accuracy? I'm sure this hasn't been overlooked. I don't even own a chrony myself but was thining of buying one and was just wondering if there is a was to calibrate one.
Jeffery
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Not a silly question at all, I was thinking about the same possibility. I don't know of a way to calibrate mine directly, but I could get a relative calibration by shooting a different gun that I'd chronied recently. Same pellets, same results, so I don't think the chrony is the problem. BTW, mine is a fairly new Beta Chrony.
Regarding "silly" questions, IMO, the only dumb question is the one you don't ask. So keep asking. Sometimes by not "knowing" something, you look at a problem in a new way. I know I learned more from the newbie electrical apprentices I had over the years than I did from most of the journeymen I'd worked for myself. Later.
Dave
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...significantly contribute to piston slam and spring fatigue? Is a choked barrel easier on springs?
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I'm sure I have read somewhere that Gamo say it is OK to dry fire their guns, though there is no mention in the manual for my CFX and frankly I cannot understand the logic, (if any). If dry firing IS OK then choked or unchoked will make no difference. Personally I will err on the side of caution and not dry fire a spring gun.