[Visser]

I'm working on a custom m4 for my brother in law which has been running for years. I did a few upgrades to it and when I put it back together it runs fine on semi but on full auto it'll do a short burst but when I try a long burst it dies off like the battery dies. Only the battery is fine and if I pull the trigger again in 2 seconds it shoots again and does the same thing.
Internally the gun has a modify cylinder, piston, nozzle, mask cylinder head and piston. I can't remember if I changed the gears or not but either way I know the shimming is great it was checked and double checked. The piston is moving forward and back freely and isn't jamming (i had to file it down so that it wouldn't). I tried changing the motor to see if that was the problem and it still did the same thing. I would consider the wiring cause it has joiners in it needed to take the stock off, but would it die off like that because of the wiring? Wouldn't it always just lack in power then?

Any suggestions please?!?!? Thanks