Personally I'd get a smoke generator and test. It would be a shame to pull apart the manifold if the problem is elsewhere. I thought it was a useful exercise and reduced guesswork.
You know I think I’m actually seeing the Venturi.. not sure if my theory is correct as I can’t see the butterfly. Maybe get a headlight and look down there.
To me it looks like the primary butterfly is open at idle, which I do not think is correct. Hopefully someone more experienced can chime in.
At idle I believe the fuel is drawn through a passage below the primary butterfly, adjusted by the idle speed screw.
Does pulling up on the pedal...
I recently had something similar happen. The paper gasket btwn the carb and base had become deformed and was holding the secondary butterfly open. Maybe worth checking, but it sounds like you also have a gasket leak to some degree...