Just make sure when you spin the gears backwards that they actually spin and arent all tight acting.
I basically unscrew my car...pull my engine back towards the gear and make a good tight mesh. Then spin it backwards this will move it forward enough to have some good spin but not too much to where it will chew up a gear.
Then you tighten it while leaning the chassis backwards a tiny bit to help the motor stay against the gear mesh. Then retest once fully tightened with the backwards spin and listening to the mesh....it should spin pretty freely.