Now that you have mounted the engine, did you also readjust the gear mesh? If you look at page 12 ,first box, of the NX manual you should see a 4 G36227 cap screws. You can loosen these screws to adjust the gear mesh.

A quick and easy way to set the mesh is to cut a piece of notebook paper and push the pinion gears as close as possible to the spur gear. The paper should come out with an accordian style fold, but it shouldn't be ripped.

That OS 18 is one fast motor.. Enjoy it!!

If the gear mesh wasn't the issue, please describe the problem in a little more detail. But I hope it was just the gear mesh.