[MaciekA]

Drop-in mechboxes are never ever actually a matter of a simple drop-in... but there is hope. There is a whole world of stuff you need to work through to get this going "nicely", however it looks like you've gotten a good start by testing the motor and electrical system. It is a good sign that the motor moves when you have it out separately.

Note that the G&P M120 motor is a fairly weak motor, so it's entirely possible your battery simply doesn't have enough juice to move it, especially if the pinion/bevel mesh isn't good yet.

Assemble the mechbox with ONLY the pistol grip attached to it so that you can do adjustments and test quickly without having to re-assemble the entire gun over and over. Make sure to pull back the selector plate to have it in full-auto during testing.

Now then, things to try:

1) Adjust motor height up and down and see if the clicking goes away
2) Rewire the gun for better connectors like Deans and/or try a more powerful battery
3) If you're willing to disassemble the G&P mechbox, go ahead and do so. Then remove the ARL + the cylinder/spring/piston, re-assemble the mechbox without them and see if the motor can turn the gears without those things (once again, do this with only the motor grip attached, it'll make debugging go much quicker).

For idea #3, that's a good time to adjust motor height and get an optimal sound. The less screeching, the better, but note that having the motor drive the gears with no load will always sound a little bit crazy since it is able to turn at full speed without any load.

Once you have this all working and the gun can fire, you should then learn how to shim properly and do that next. G&P drop-in mechboxes are shimmed "okay", but you can do a better job just by following some tutorials and asking questions here.

Good luck... Let us know how it goes.