AEG turned semi only on it's own
 

Ok... now, This problem confuses the hell outa me. (I haven't gotten around to going into the mech box yet due to lazyness) My JG 416 (only internals still JG are the cylinder/piston parts) Has randomly stopped firing auto. The gun worked fine, then sat still without me playing with it for a few weeks and now it won't fire in auto. I only noticed the problem when I picked up the gun to play with, switched the selector to semi (without a battery) and pulled the trigger then did the same on auto and it clicked.... put a freshly charged battery in and on auto it would fire in this sequence with each trigger pull: semi, auto, semi, auto, etc. Next day I tried it again to show a fellow airsofter and the auto feature turned into just semi with a 10x heavier trigger pull. Tried again the next day and everything worked flawlessly. Tried again today while i was talking to crunchmiester on eve about it and it was doing the same (all along the semi setting worked flawlessly). Started taking it apart, got down to just the mechbox/lower receiver/grip, plugged in a battery and auto worked perfectly I figured what ever it was musta solved itself, put everything back together and the problem started again.... took it down to just as it was when it worked, no luck the problem was still there. I think it's a fraked up selector plate but what else should i look at when i have enough time to go into the box?

Use a calibrated hammer, guaranteed to never break again.

Honestly I'm unsure of this, mechbox design is full auto in its natural state, semi is a forced mechanical action. Sure as hell a Trigger Master will fix this for good, but still, odd issue.

You're lucky.

My RPK refuses to go into Semi auto until I slap it up-side the receiver a few times, and call it dirty dirty names.

My only suggestion would be that.
Slap it a few times, talk to it dirty and try again.
If that don't work, try rubbing it and talking to it ever so nicely. You may have to talk extra nice to make up for the dirty talk, but who knows.

I had a simular electrical gremlin problem with my ics m4. In the end it turned out to be a worn cut-off lever problem. (Hope that gives you a couple ideas)

>.> i'm hoping for it to just be a worn cutoff lever or broken selector plate.. else wise I'll be stuck yelling at my gun for a while Lol, Thanks everyone for your responses.