Here's a late thought. Has anyone had experience with the Marvel carb. one piece venturi and the type of fuel nozzle used. My carb. is a 10-4894 that has the one piece venturi mod, but retained the original fuel nozzle. Precision Airmotive SB MSA-7 only recommends the newer nozzle when roughness, caused by a rich mixture, is detected. As I found, leaning the mixture made no change, however there is an increase in engine vibration above 2300 RPM.
I did experience the richness problem in a C-172 (O-300) back when the one piece venturis were first mandated. In that case the richness was easily recognized.

Whadda ya think ?