First AEG, first upgrade

[One Stray Bullet]

Hey everyone. I recently purchased my first AEG, a King Arms M4, from explodingpandas on ASF. It currently has a PDI 120% spring installed, but I believe that a compression issue is causing it to shoot lower than it should be as its only at about 290 fps. I'm not sure how exactly to go about fixing that, but I have friends that can help and thats not really the point of this post.

I want to get it up to just under 400 fps, which is the limit at the fields I will be playing at in the SF bay area. I purchased an M120 spring to this end. Basically, I want to install it myself for a few reasons:

Cost: I know it doesn't cost a ton of money to have a shop do it, but I just dropped a decent chunk of money on the rifle as well as some accessories, and being a college student every dollar counts.

Experience: I understand the basic way that an AEG operates, but I have no experience working on internals and I would really like to have the ability to work on my own gun if need be. This will also allow me to be more self sufficient and diagnose problems when something goes wrong, as well as perform other upgrades in the future.

Is this viable? I've watched a couple of tutorial videos detailing how to go about it and it seems like something thats within my abilities, but I don't want to evaluate this wrong and end up trashing my gearbox. I asked a person at the the local airsoft shop about the level of difficulty and he said 'If you have to ask that, you can't do it'. I may have phrased the question to him wrong. I understand that it's more involved than simply tossing the spring inside the case. I consider myself to be a fairly level headed and careful individual and my father, who has a decent amount of mechanical expertise, is willing to assist me. I'm more computer inclined than mechanically inclined, but I really want to learn.

Anyways, if you could offer any tips, suggestions or advice that would be great. Even if its 'Don't attempt this, you'll likely trash your gun', I want to hear it! Thanks in advance

[Jewish Ninja]

Watch the videos a couple times and if possible, watch someone else do it in person. I figured out how do it myself from watching my teammate take a GB apart in person.

That being said, changing a spring is pretty easy. If you leave everything in place, you shouldn't have to worry about it all getting messed up.

[Pancake]

The way I learned I just opened up my gearbox and took everything apart, then I watched a guide on how to put it back together and instead of my normal parts I just put in my upgrades.

It was actually a lot easier than I thought it was, only thing that was hard was putting in the spring, anti-reversal latch, keeping the wiring inside its grooves, and closing the gearbox with everything put in right.

Just make sure that you don't lose any of the small springs or parts, and that you use a small enough screw driver to fit through your spring guide to hold down that spring.

Good Luck!

[One Stray Bullet]

Thanks for the advice guys, this is encouraging. I'm going to give it a go tomorrow and I'll let you know how it goes.

For a secondary question, where do compression issues occur and what should I look for?

[Pancake]

Thanks for the advice guys, this is encouraging. I'm going to give it a go tomorrow and I'll let you know how it goes.

For a secondary question, where do compression issues occur and what should I look for?

At the Hop Up bucking, Cylinder Head, Air Nozzle, and Piston Head.

To check your compression, just put the Cylinder set together with the air nozzle attached to the Cylinder Head. Then put your finger on the opening at the end of the air nozzle, and try to push your piston through your cylinder. It should go most of the way, then become really hard to push forward.

The the hop up bucking, I don't know of any way to check compression, put to be on the safe side just throw some teflon tape around it and the opening of your hop up.

[RiotingCows]

If your changing a spring I would advice having a second person there to assist.
I don't know if I just haven't figured out the trick yet or what but the newer springs tend to be a little long and a little stiffer making it difficult to get inside the gearbox with one hand without having all your gears fall out of alignment and anti-reversal latch flying off.

[One Stray Bullet]

Alright well I installed the spring and reassembled my gun. When I went to shoot it fired about 10 times and then stopped. It made a sound like the motor was attempting to turn over but was stuck. I tried firing two more times, it unstuck and then fired about another 10 shots before the same thing happened. I can feel the motor straining but it won't turn. When I took it out of the grip it was pretty warm, which is what I expected with it straining like that.

Could this be a problem with the motor height? The gears appeared to spin freely before I put the spring in. I didn't change any of the shimming. All I did when I was changing the spring was take apart the gearbox, remove the tappet plate spring, remove the cylinder and piston, change the spring and reinsert all of it, then tighten the box back up and reassemble the gun. The motor wires were giving me some trouble, but when I got it in it appeared to be moving freely up and down on the spring.

The anti reversal latch also gave me some trouble as it didn't seem to want to sit straight, but I believe I got it back in correctly as well. It is supposed to fit into the notch on the bevel gear and prevent it from spinning backwards, correct?

[pseudoanimosity]

How powerful was the spring you installed? It could be that the motor isn't powerful enough to pull it back. You may need to upgrade your gears, or clip a few rings off the spring so it will work.

[One Stray Bullet]

I got an M120 spring. I tried assembling it a second time and I could hear the motor grind against something. I took the rifle apart again and found that the air nozzle was locked in the rear position, which makes it seem like the tappet plate is getting stuck above the sector plastic piece on the sector gear instead of being pushed backwards by it.

Edit: I thought about this and realized that it would be stuck back anyways depending on the position of the gear. During my third assembly and test it cycled and fired once and then the motor started making a high pitched noise similar to an electric drill. However, it was indeed spinning.

Update: It appears to be fixed. I switched a shim that I thought I had put on the wrong gear and have fired ~100 rounds without issue