RE: Eflite Blade CX
Take out the motor that drives the 'tight' rotor, then turn the rotor to see if it is still tight. Of course, it will always be easier to turn with the motor out, but it should turn VERY VERY freely with no motor. If you feel any resistance at all, then the problem is not gear mesh.
If it turns very well with no motor, AND you are SURE the motor is good, then put a piece of paper between the motor gear and the main gear when reinstalling it. Push the paper down into the teeth between the gears, then tighten the motor down. Once it's down, rotate the gears to 'walk' the paper out.
Now, your gear mesh should be just right, because removing the paper will leave a slight gap between the gears.
You may have to try it a few times until you get it just right. Be patient. If you get frustrated, just set it all down, and go do something else for a while. When you come back, it will be MUCH easier to get it right.