Thanks, Don, this is something I have always puzzled over, having read texts on fluid mechanics in my student days. I set fixed venturi control line engines up a long time ago with the spraybar hole at right angles to the airflow, with what I thought were very good results, but not measured by a tacho in those days.

For engines equipped with an RC carb, maybe the 'manufacturer's setting' with the hole pointed roughly towards the crankshaft, ie 'downwards' is a compromise to give acceptable results at various positions of the throttle barrel. Who knows?