m4 only fires properly after about 15-20 shots

[fronugget]

Hey all, I have an Echo 1 troy mrf-c M4 with a newer spring and a stronger air nozzle. Recently the gun would blank fire, shoot 2 bbs, or shoot 1 properly, until about 10-20 shots. After that it would shoot completely fine. I've lubricated my mags (mid caps) and they seem to all be working perfectly. Does anyone know what could possibly be causing this problem?
Thanks

[ThunderCactus]

Have you tried under-loading the mags?
If mechbox work was done, did the tech test for proper compression?

[Zack The Ripper]

I second under filling the mags. If they hold 120, fill them with 90 and see what happens. I have several mags that I have to do this to with my modded M4 as the spring tension in the mags is too high.

Should this technique work, them it is likely the spring tension is too high. If this is the case, you can try filling the mags to capacity and leaving them filled for five days or even more. This extended compression of the spring follower should allow some of the tension to release and should help with feeding. This does not always work though. Again, test the above mentioned method out of only filling the mags 75-80% and see what happens, then go from there.

Also, be sure your hop up is very clean inside and out, and ensure there isn't an excess of lube. This will be detrimental to feeding as well as proper air compression when firing.

[fronugget]

I tried under filling my mags, but the gun would only function properly if I loaded with a maximum of about 50-60, while they're advertised at being able to hold 120. This originally wasn't a problem. All gearbox work was done by myself. They're aren't many available techs around where i am, if any, and although I am no profession, I do (atleast somewhat understand what is going on inside the gearbox). I'm gonna try to load the mags to their max capacity for almost a week and see what happens.

[ThunderCactus]

Test your mechbox for proper compression.
Some air nozzles don't work well on some cylinder heads. I went through about 5 to find one that sealed properly on my old G&P M4.
I've seen loses as large as 40fps from a bad air nozzle seal, that would definitely cause feeding and firing problems.
There's also a chance, although rare, the mechbox is misaligned, causing an airleak.

Feeding issues are usually difficult to narrow down, but you said it was working well before you opened the mechbox. So it sounds like it's either an alignment issue or a compression issue.

[lurkingknight]

take your tappet spring, clip off the loop that holds onto either the plate or the post on the mechbox... take needle nosed pliers and bend out another two loops to replace the piece you just clipped off.

This increases your tappet return tension vs the strong spring of your mags, sometimes the mag spring can disrupt feeding.

What brand BBs are you using? some brands don't roll as well in the mag and cause feeding issues with more BBs in a mag.

Also list all the work you've done in the mechbox since the last time it fed properly.

[fronugget]

I think it must be a mag/bb problem. When I first got my mags I was able to fit 120 in them no problem (like one small speed loader and almost a half). But now I'm struggling to put in 80, even after lubricating them.

I haven't done to much in the mechbox, i've downgraded the spring and replaced the airnozzle.

I'm using velocity arms 0.25g's from Toronto Airsoft and I'm not sure if it's been missfeeding ever since i've been using these. It's possible that before I was using 0.20g's when my mags were feeding fine.

Also, something else to point out, bio bb's (atleast the ones from walmart, lol) do not function whatsoever in my mags. They all just jam in and dont come out even when trying to release them. I dont know if this has anything to do with my current problem, but anything helps I guess.