I have a 7.5HP Fleetwin that has similar issues. I was about 99% sure it was a carburetor issue but when I swapped carbs from a great running Fleetwin nothing changed. Mine runs “ok” above idle but seems incapable of drawing fuel through the idle circuit of the carb. Frustrating. Maybe your solution will be my solution.
One thing I could suggest you try: Have a close look at the needles. Are they the correct ones? I once had a 12HP like yours that ran terrible and when I swapped carbs things improved. Looking at the original carb I saw that someone had replaced the slow speed needle with an incorrect one. It screwed in but was not long enough and was of the wrong taper. Hard to tell without another carb to compare against as they have no part numbers.
