Krazy what’s the carb p/n#? New and old same thing?

Why are you choking with ignition off and then flipping the engine? I think this is unsafe choke and flip with ignition on, the engine will sputter and die when enough fuel is present to fire rather than load up and flood with ignition OFF.

Tim mentioned the best method to diagnose your dilemma IMO.

Some of these carbs high speed needle affects low end operation. You might experiment here too.

Did you remove the cover plate to install a vent line tube barb or similar?