You said it has lots of blow-by. So, with the hose connected that was
going into the carb. That blow-by is not air, it spent combustion gases
with no oxygen. So, removing that from the intake air flow would change
the mixture and that in turn could cause it to stall. That's a plausible
possibility.

Yeah that's the only thing that makes sense unless there is some passage from that connector to the intake end of the carb but I doubt that's the case.. BUT the countervailing argument is that the carb is not adjustable so it's not like it was adjusted with substantial blow-by taken into account.

I actually don't know if blow-by is excessive since I've not compared to others. As a single cylinder engine I know with the volume behind the single piston undergoing constant and considerable change I figure there will be a lot of movement in and out of the breather. I don't know if there is a one-way valve that causes only crankcase output to go to the carb or if it breathes back in that way as well. That would give it the benefit of filtered air.

I have to say, having grown up on one Tecumseh-powered mower after another, I miss adjustable carbs. In fact I'm surprised they work at all since no two engines or carbs are precisely the same so whatever mixture is built into the fixed carb cannot be as optimum as adjusting for the best.

IDK. Does the governor move at all in response to load?

No, not seeing any movement. AND the kicker is that IIRC the throttle should go to wide open when the engine is stopped. It does not. Just stays against the minimum stop.

It does bog down a little in heavier grass but not as much as I would have expected. While I have not measured the speed, I've got the limit screw for min throttle set for a more or less normal sound when it's above concrete with no load. Maybe a bit on the fast side but if so, not by much.