1972 CB350 Twin: Bike has existing electronic ignition/ timing looks bang on/ fires on right cylinder/ has spark on left cylinder but no combustion - why?

Left Keihin carb looks very clean.  Pulled jets & emulsion tubes from carb & sprayed with carb cleaner then air.  Inside of carb is spotless.  Idle speed screw looks great & nothing is plugging channel to main carb body.  Why won't left side fire up?