An interesting new glitch has developed since I installed the spacer behind the spur gear. When increasing throttle, there's a high pitched squeal, much like you'd get if the pinion were rubbing the inside of the gear case, or if you've got a worn pinion. Noise is not apparent when off throttle. I backed off the motor screws a tiny bit and the noise went away. I thought, because of this, that maybe my pinion was out too far, so I knocked it in a teensy bit. Same situation.

Seems a bit weird. Maybe an alignment issue in the gearbox? I'll be trying a few things, including a new pinion gear. Any thoughts?