"(2) Looking at the YF, that same passage that vents to the exterior, also has an outlet to the interior. I really wonder if the exterior outlet is even necessary. But the engineers put it there, so I will leave it alone."

Bowl vent.

Re-read "carburetion 101, lesson 3" about atmospheric pressure.

Jon

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop