Rotor spins but it won't lift off or the rotor won't spin? If the motor isn't turning check to make sure throttle at 0 before you start and that idle up is off. If it won't fly but is spinning up to full throttle then obviously you have a pitch problem that is keeping the heli from making enough lift to fly. Did you try to adjust tracking or something of that nature? A little wierd if all you did was replace parts, then you shouldn't be out of pitch range. Did you put the blades on correctly and not backwards? When viewed from above the rotor spins clockwise.