Two round burst
 

I'm not the greatest with gearboxes so hope someone can help me.

I decided to reshim my gearbox but after I put everything back together. It started to fire in two round burst in semi time to time. Now it shoots fine but maybe like every 20 shots or more, it fires in two round burst. I was thinking if there was problems with the switch assembly but maybe it's my shim job?

I'm using a 9.6 NiMH battery.

If your sector gear is shimmed too low (to the left), it can interfere with the semi-auto cutoff lever. Larger diameter (not thickness) shims on the bottom of the sector gear can also interfere with the cutoff lever.

tunabreath got it right !!

I've heard of cutoff levers getting worn down on King Arms M4's.... Could that be it? Basically it gets worn down and doesn't always cut off properly.

If sector gear shimming is correct try replacing with a known good cut off lever and if it works flawlessly after a stress test then that's your problem.

Maybe I shimmed the sector gear too low.. not sure yet but I will check tomorrow. IIRC, it didn't had the burst in semi issue before I re shimmed it.

If it's got a weak spring this can happen. friend of mine had this "problem" with his AK, was just cause it was a brand new powerful battery, after a few games it went back to normal as the battery broke in. Though if you have anything above a M90 spring I can't see this happening.

I have a 110 spring in my CA and it did the same thing... I switched to a lower voltage battery and it stopped.

I have a M90 spring installed in it but I was only using a 9.6v NiMH battery...

This is an overspin issue.
The inertia due to the soft spring /high power motor makes the gears continue moving, despite the motor is not reciving energy (cut of lever works well)
This can be solved:
1) using lighter gears, like systema energy done in carbon.
2) stiffer spring
3) less battery voltage
4) (what I choose an the best with springs < 110) Mosfet with active braking.