Well, I just upgraded my TM M4 with a new spring, new bushing, new piston head, and new piston. I put the mechbox back together, put it in the gun, put the motor back in, plugged in a battery, pulled the trigger, and it made a relatively weak turning sound then stopped. I didn't pull the trigger again to avoid messing up the gears. I havn't re-opened the mechbox yet to look at it, but does anybody know what can cause this?
It seemed like it ran long enough for the gear teeth to catch the piston, then stopped.. so maybe it can't pull the spring back, or just a gear problem?