That's good in way that you have proved it is a carb problem.
Better than a manifold or cylinder problem.
It does seem odd if you have blown the passages clear and can't see the reason, I'd strip two carbs and do the same compressed air test on both, I'd also blast some fuel through the passages so you can see the flow discharge.
Swap the idle jets between carbs as well.
Don't get fuel in your eyes, it hurts like hell.
If none of the above helps, take the offending carb back to your supplier and see if they can help you.