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What did I do wrong? Bucking suggestions! Prometheus Tightbore & M120 spring


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#21 vanevery

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Posted 04 July 2013 - 07:40 AM

The G&G Green bucking has a large bucking mound. It takes a while to break in. So, if you are using bb's less than.30's, you my experience over-hop. I use .30's with my G&G Green buckings and so I do not even adjust for any hop at all until after 40 yards.

Edited by vanevery, 04 July 2013 - 07:41 AM.


#22 EzDeezer

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Posted 04 July 2013 - 11:14 AM

Yeah I would only use atleast .30s with the G&G as Van said, but Im surprised you got the prometheus to fit with the G&G green as well.  This is something I will now have to try out, Ive had success with my KM barrel but would like to see if the prometheus is any better for a long time I have been kept away from prometheus barrels by the fact that many people say they do not work well with KWA's hopup unit, if it is working well for you, then that is a sign that it should for me.

Edited by EzDeezer, 04 July 2013 - 11:15 AM.


#23 rawfuls

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Posted 05 July 2013 - 11:15 AM

I am using .25g bb's, maybe I'll order a few .30g bags.

As far as the compatibility issue, yeah, it seems to be working out flawlessly for me ,now.
I don't have a chrono to compare against my previous readings, but it definitely feels a bit stronger (as far as M120 spring and the air seal is concerned).

Of course, when the retaining clip, there was a little excess of green bucking- my guess is that without any oil nearby (I did it at a friend's house), it didn't slip in all the way [but on the otherside of the hop up unit, it was where it should've been].
So although I'm not a pro, I'd say it worked out very well.

#24 EzDeezer

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Posted 05 July 2013 - 11:23 AM

Well Im about to try a prommy barrel in a couple days when mine comes, hopefully to good use, I will avoid using the 2Gx bucking with it though as that seems to give a lot of people problems.  If I get it to work maybe ill throw out a comparison of it vs the KM head that alot of KWA users seem to love.  I don't expect one to be much better than the other but you never know, and barrels deviate one batch to the next but it will still be interesting to see the results however accurate or innacurate they may be.

#25 rawfuls

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Posted 14 July 2013 - 12:04 AM

Well I'm definitely doing something wrong.
Finally fielded it, it chrono'd 283.7
The field refs didn't think their chrone was working since it was so low, they pulled one off the shelf-- also read 284.0
Now obviously, something wrong. I have a feeling it's within my bucking and air nozzle setup since that's where I had the most trouble. I may just bring it to the local shop and have them take a look at it and give me an estimate before working on it...

What should I be looking at, price range of getting this fixed?
Prom my tightbore and 120 spring not even breaking 300? Something's wrong!


#26 vanevery

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Posted 14 July 2013 - 07:03 AM

View Postrawfuls, on 14 July 2013 - 12:04 AM, said:

Well I'm definitely doing something wrong.
Finally fielded it, it chrono'd 283.7
The field refs didn't think their chrone was working since it was so low, they pulled one off the shelf-- also read 284.0
Now obviously, something wrong. I have a feeling it's within my bucking and air nozzle setup since that's where I had the most trouble. I may just bring it to the local shop and have them take a look at it and give me an estimate before working on it...

What should I be looking at, price range of getting this fixed?
Prom my tightbore and 120 spring not even breaking 300? Something's wrong!
What BB weight are you using at chrono?
Did you have the Hop-up turned all the way up?
How does the gun actually shoot?


First I would remove the Inner Barrel and Hop-up Unit.
Dis-assemble the Hop-up unit.
http://www.kwausa.co...p?showtopic=493


Clean the Inner Barrel, Bucking, and Hop-up Unit thoroughly.
(Don't misplace the Nub).
Trim a little off the tale end of the Bucking as to not interfere with the C-clip


Re-install everything.
1. Place bucking on Inner Barrel.
2. Install barrel and bucking into Hop-up Unit.
Hop-up Unit should be dis-assembled at this step.
Should look like this:
Posted Image

3.Then, re-assemble the Hop-up Unit with the Inner Barrel and Bucking already inserted.
Insert the Chamber Spring and then place the Nub in its place holder on the Arm.
Close the Arm and test it by applying pressure to make sure the Chamber Spring is working properly.
Also look thru the Hop-up and make sure the Nub is centered as the pressure you apply is raising and lowering the Bucking Mound.

4. Place the Gear Wheels and Star Locks back on.

Try this and let us know how it works out.

Edited by vanevery, 14 July 2013 - 07:04 AM.


#27 rawfuls

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Posted 14 July 2013 - 11:00 AM

Using .25s for my mag, field refs used their own mags and weights for testing, but at .25 it shot 283.

Hop up is all the way off. Still overshot a little....

Honestly it felt like it shot harder, but may very well have been placebo effect.

I unfortunately don't have access to a chrono so I'm contemplating dropping it off at the shop or trying my hand at it.. Again..

Edited by rawfuls, 14 July 2013 - 11:07 AM.


#28 EzDeezer

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Posted 14 July 2013 - 11:18 AM

Did you try out other barrels/buckings?  If these get the same results, it could lie within your internal compression (piston head o-ring, cylinder head to cylinder seal)  Something could just have easily gone wrong when installing the spring, it certainly cannot only be the barrel and bucking.  Test what you can and eliminate suspects until you find what is behind your problem.  You also say you are using the KM4 CQB, but extended the barrel.  The cylinder in the KM4 CQB is different than that of the regular KM4 series weapons, and since you extended the barrel, that just as well may be the cause of your FPS loss.  If you can, install a regular KM4 cylinder or a regular Type 1 cylinder.

Edited by EzDeezer, 14 July 2013 - 11:36 AM.


#29 rawfuls

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Posted 14 July 2013 - 05:34 PM

I'm going to take the bucking apart and use oil this time to get the sucker in, as opposed to forcing it in.
Are there any 'super' high res pictures of the Piston Head ORing, Seals, of the gearbox?
I definitely suspect the gearbox being the issue here mainly because I feel that a bucking can in no way lose ~100fps due to an air leak (that's just me..)

However, will definitely take it apart.
The local shop isn't open Mon-Tues, so if I can figure it out prior to Wednesday, I'll probably take it in and have a pro take a look at it.
Bummer, this is..

When I said I extended the barrel, it's only extended by about 2 inches, into a standard M4 length.

#30 EzDeezer

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Posted 14 July 2013 - 05:42 PM

Every couple inches is actually a big difference when matching cylinders, the KM4 CQB barrel is usually a 300mm if I remember, whereas the barrel you installed is a 363mm, thats somewhat significant and could effect it, but maybe not that much, I would try out another 363mm barrel and see if that does the same thing.  The barrel should not be sticking out of the back of the hopup unit or out of the back of the bucking, the bucking should still fully encase the barrel when inside of it, with the lips a few mm past the barrel if that makes sense.  The barrel should also be in far enough to where the grooves for the c-clip to attach are just showing.

#31 gcw360

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Posted 14 July 2013 - 06:42 PM

I just rescanned this thread.  You were at 360-370 and now you are down to 283 with .25's.  That would be about 315-320 with .2's

Regardless, lets assume all the measurements were taken with .25s......I have seen more than one KWA loose FPS with a swap to a G&G green even with the stock barrel. For instance, a stock SR10 firing 400 was reduced to 335FPS with a G&G green bucking.  There can be a number of reasons for this:
  • Air leaks
  • Occluded inner barrel
What's important is performance on the field and FPS is not the end all be all.  The drop you are seeing is not all that far off from what I have seen in other KWAs with a G&G green added.  Thing is, the Promi barrel may be adding to the effect.

#32 rawfuls

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Posted 14 July 2013 - 06:49 PM

Instead of trying to keep explaining here are the pictures.
Shall I install the stock barrel back again and see?
I still do believe it may be something with the piston/cylinder that I could have possibly screwed up when installing the spring- but being a newbie to gearboxes, not sure what I would be looking for to tell if I did something wrong- or not.

Posted Image
Posted Image
Posted Image

#33 gcw360

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Posted 14 July 2013 - 06:58 PM

Personally, I would instal the reinstall the stock barrel and bucking.  If the FPS isn't up to a reasonable level then I would open the gearbox to see if there is anything amiss inside. It may be as simple as replacing the piston o-ring with a new Danco #14 0-ring.  It could be a stripped piston though I doubt it.  It could be a raked cylinder head. That's an easy fix and is easy to diagnose.  If that's the case, it will manifest itself prominently in short order.

#34 rawfuls

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Posted 14 July 2013 - 07:01 PM

View Postgcw360, on 14 July 2013 - 06:42 PM, said:

I just rescanned this thread.  You were at 360-370 and now you are down to 283 with .25's.  That would be about 315-320 with .2's

Regardless, lets assume all the measurements were taken with .25s......I have seen more than one KWA loose FPS with a swap to a G&G green even with the stock barrel. For instance, a stock SR10 firing 400 was reduced to 335FPS with a G&G green bucking.  There can be a number of reasons for this:
  • Air leaks
  • Occluded inner barrel
What's important is performance on the field and FPS is not the end all be all.  The drop you are seeing is not all that far off from what I have seen in other KWAs with a G&G green added.  Thing is, the Promi barrel may be adding to the effect.

I will be returning the tightbore shortly, as the effects of accuracy aren't too important to me.
I'll be reinstalling the stock barrel with the G&G bucking (until KWA gets more 2GX buckings in stock).

When you noted about the FPS drop being somewhat normal, the 360 was with the stock spring, whereas the new 280 reading is with a new Prometheus 120 spring.

#35 rawfuls

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Posted 14 July 2013 - 08:07 PM

Just finished putting the stock barrel back in, cleaned it (since no cleaning rod or patches, I shot a Qtip down the barrel, just fattended it up, and blew down the tube a couple times.
Definitely cleaned it up. Made sure it was spotless before putting it back in, and shot some compressed air down it to remove lint or hair)

Cut the bucking a sliver so it wouldn't protrude out into the retaining clip, used some shock fluid for RC cars on the exterior of the bucking to slip it into the hop up unit.
Looked flawless to me.

Put it back together and did the quick poormans chrono- won't even pierce the side of a coke can.
Dented it... but no pierce. (with 0.20 and 0.25g bb's at 3inches away from can)

Sounds like the gearbox?

Edited by rawfuls, 14 July 2013 - 09:00 PM.


#36 EzDeezer

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Posted 14 July 2013 - 08:57 PM

Yeah, it should shoot through the side of can signalling at least 300 FPS I believe, just might as well be your cylinder head or piston head oring, you're gonna have to open it up and check it out it sounds like.  I would not blame the barrel or the bucking, the G&G green may actually increase FPS due to its seal (in my experience atleast, not sure what gcw has seen), and since you are using the stock barrel now it cannot be the prommy barrel.  There is even the case that your spring is a lemon or something, but I dont see that being possible as prommy has very good quality control, and it is hard to mess up a spring (unless its made by matrix), perhaps the spring was installed incorrectly, but that is also very unlikely.

On another note, you are putting the bucking on the barrel, lining up the ridge on the bucking into the slot on the barrel, and then sliding it in, then making sure that the lips are correct when in the hopup chamber (barrel should not be sticking out of the bucking at all, lips should be making a complete "circle" at the back of the unit (this is hard to explain without pictures and I do not have any, post a picture of the back of the hopup unit (where the nozzle inserts) if you can with everything installed and I can tell you if it is installed correctly)

Edited by EzDeezer, 14 July 2013 - 09:12 PM.


#37 rawfuls

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Posted 14 July 2013 - 09:12 PM

View PostEzDeezer, on 14 July 2013 - 08:57 PM, said:

Yeah, it should shoot through the side of can signalling at least 300 FPS I believe, just might as well be your cylinder head or piston head oring, you're gonna have to open it up and check it out it sounds like.  I would not blame the barrel or the bucking, the G&G green may actually increase FPS due to its seal (in my experience atleast, not sure what gcw has seen), and since you are using the stock barrel now it cannot be the prommy barrel.  There is even the case that your spring is a lemon or something, but I dont see that being possible as prommy has very good quality control, and it is hard to mess up a spring (unless its made by matrix), perhaps the spring was installed incorrectly, but that is also very unlikely.

As far as inspecting the piston and the head, this is the cylinder in which the spring resides in right?
As you can tell, I've never gone any further than the spring in the gearbox, hence my hesitance of diassembling and inspection.
Is there a tutorial (preferbably picture and written?) of the piston and head and the entire gearbox?
I have a few videos on youtube, but they're not of the greatest of quality of which I can compare my gearbox to theirs to ensure proper setup...

I'm guessing lots of small springs and gears I can potentially lose..

#38 EzDeezer

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Posted 14 July 2013 - 09:47 PM


this is a decent video on how to disassemble it all.
The spring is on the spring guide at the back of the gearbox, the tighter coils of the spring should be at the backside of the gearbox, the spring then goes inside the piston, which goes inside the cylinder (the brass tube).  The piston should be on the rails on the side of the gearbox.  The cylinder head is the part with the nozzle on the front of it, it should line up onto pegs which go into the holes on either of its sides, and should be pushed into the cylinder just far enough to make a seal and allow it to fit into the gearbox (should not be able to see the oring on its side)  The piston head is the round flat piece of aluminum on the front of the piston, it will have an oring around it, to improve compression you can replace this o-ring with a danco #14 as gcw has stated, these can be picked up at a hardware store, stretching them out a bit will improve the compression further.

Edited by EzDeezer, 14 July 2013 - 09:50 PM.


#39 rawfuls

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Posted 14 July 2013 - 10:02 PM

"tighter coils should be at the backside of the gearbox"
on the spring guide, right?

Could the spring be backwards be causing this issue?
May...or may not have put the closer ones toward the piston..
By tighter coils you mean this part:

Posted Image

Edited by rawfuls, 14 July 2013 - 10:02 PM.


#40 EzDeezer

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Posted 14 July 2013 - 10:55 PM

Tighter coils on the spring guide, yes those are the tighter coils.  I do not know the effect of having the spring on backwards, but I assume it would not be a good thing.

Edited by EzDeezer, 15 July 2013 - 11:47 AM.





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