ORIGINAL: JockC
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?
Excellent point.
The air flow through the barrel in relation to the spray bar changes from WOT to idle. At idle or so, the flow would be at 90 degrees to the slit, while at WOT, 180 to the slit. A guess on my part, this would give a better atomization at lower RPMs. Now, why doesn't the Walbro carb do the same? One possible answer is that it is designed for WOToperation, IEleaf blowers, chain saws, etc, which are seldom operated at mid range.
Don