Low FPS help!

[Ożga]

Hello all, a couple months ago I had my M110 spring (I'm assuming this was the stock spring) swapped out for a M100 spring to make my gun CQB legal. Before this swap, my gun chrono'd ~375fps, after the swap it chrono'd ~275fps. I was expecting somewhere in the 330-340's range. I didn't make a deal about and left it as it was for about 2 months. Then 2 weeks ago, when I went to a game my gun chrono'd 215fps! I just opened up the gearbox on my own this time expecting to find a broken air nozzle or something along those lines, but everything appeared intact. I don't have a chrono on hand and I only do so at games. So I don't know if opening it up and putting it back together would have solved the problem, or maybe the suspected leak is where the air nozzle goes into the hop-up chamber. The gun in question is a DBoys RK-05 (AK-74m) with a 7mm bushing Version 3 gearbox. I plan on ordering several parts to make it high speed, but for the short meantime, do you guys have any ideas as to what is going wrong with my gearbox? I just don't want to order parts and completely miss the part required to fix this problem.

[T6e9a]

Make sure your cylinder is in properly. Any holes or ports should be in the rear. Also make sure there is decent compression.

You test it by putting a finger over the air nozzle from the cylinder head, with the cylinder attached, and then push the piston in. If when you push it in, it can't move, or barely moves, it indicates that little to no air is escaping where it shouldn't. But if the piston easily goes all the way in, with no resistance, you will probably want to swap out your O-ring, and lube it up.

[Ożga]

I checked the compression when I opened it. There was decent air compression and nothing seemed unusual.

[T6e9a]

Checked both with an without the air nozzle?
Barrel and bucking were not messed with or touched/changed during any time?
And did you have the springs swapped out at a certain place/business, or did you do it yourself?

[Ożga]

I checked with the air nozzle, but not without. I never touched the hopup and barrel assembly, I stilled haven't checked to see if my hopup bucking might have torn. I got my spring swapped by one of the Kalamazoo airhouse techies.

[Deuce]

Sept 15, 2013 8:29:54 GMT -5 Ożga said:

one of the Kalamazoo airhouse techies

Techs should know what they are doing, but nobody can vouch for them unless they personally know them or has seen their work. With that said, check to see if you have a cracked cylinder head, air nozzle, cylinder, ect. in the vital parts of the gearbox for compression (don't go checking the gears, anti-reversal, or other parts). I have personally seen some techs drop parts and/or throw them around on their bench which can cause burrs, cracks, chips, or even severed O-rings/seals.

[Ożga]

Like I said, I checked all of those components and found no signs of cracks or extraneous wear. I just checked the hop-up components and they are all perfectly normal. My guess still seems to be that the air leak may be located where the air nozzle meets the hop-up chamber, since both parts are the stock and made of plastic.

[Bishop (Task-Force Nightmare)]

Try putting the spring you took out back in and see what happens?

Sent from my SCH-I535 using proboards

[Ożga]

I just upgraded my gears, motor, cylinder body & head, piston body & head, air nozzle, hopup rubber, and barrel. Now with a systema m100 spring, I'm getting 252 fps. The fps is very consistent but is still 90fps lower than I'd think it would be. By process of elimination, I'd put the blame on my hop-up chamber. Am I right in saying this?

[ctres]

No. You need to check your compression. You cannot assume that "upgrade" parts are not the problem. They can be just as prone to issues as anything else. You seem to have a bad compression leak, so test all air seal parts and find the leak. Once you find it the fixes are usually quite obvious but let us know if you need some ideas.

[Ożga]

Also, I forgot to mention that when one of my mags is almost out of ammo, the gun starts to double feed and sometimes the bb will practically roll out of the barrel. That is why I suspect my hop-up chamber. I checked my compression up to the air nozzle and it felt fine.

[ctres]

If you have confirmed that compression within the gearbox is good then yes, move on to the hop up system. Your symptoms sound like an issue between the lips of the bucking and the air nozzle. I have had the double feeding issue with guarder clear buckings where the lips allowed bbs to slip past too easily rather than only the nozzle chambering a round. I would say to try a cheap bucking like a systema or a lonex. Really most brands should work, even if it's not your ideal long term choice, just give something else a try to see if your problem goes away. In my instance even a stock jg bucking fixed the issue.

[Ożga]

I'll try one of the stock buckings I have laying around tomorrow. Thanks for the advice.

[Kjones]

I know someone said it earlier but you did not answer the question. Is your cylinder ported? ( has a hole in the side of it) if so then check to make sure the hole is towards the rear of the gearbox and not closer to the front. If this was the case then you would be sending the wrong volume of air down the barrel and giving low FPS. If that checks out, then my guess is the hopup.