Here are some things that may be wrong:
- Are you using the same pitched pinion as the spur? (48 pitch spur meshed to 48 pitch pinion)
- Just to get it out of the way, you are loosening up the motor mount right?
- usually with a 72 tooth spur, you can't really run any smaller than a 21 tooth pinion. (that is probably your problem)
- Put the grub screw in the pinion and tighten on the flat "key" on the motor shaft and you should be ready.
- set the mesh at first until you get used to it with a piece of typing paper to get the perfect mesh.
You may have to switch to a smaller spur to fit in a "larger pinion"
Hope this helps...