When you blow down through the 'plugged' circuits, does carb cleaner spray out anywhere? It should spray out the 3 small holes right at the carb butterfly as well as the air bleed in the carb top. In any case, you can remove the pressed-in slow jets and that allows for better cleaning access. The procedure for doing that is well documented here if you do a search, or let me know if you want to do that.
If you have separated the carbs, you can dunk the problem ones in carb soak for a while to see if that helps, or try heating the area where the passage is plugged with a propane torch. You can also try boiling brake fluid down through the jet and that will sometimes clear a blockage.