You might be right. Maybe an air leak around throttle shaft that only presents at certain levels of opening.
Or it might be some oddity in how the air flows around the choke or throttle plate. Gonna take some more figuring and head scratching.
The reasons I’m not fully persuaded that it’s a “true” vacuum leak (of manifold vacuum): carb spray test doesn’t indicate one, vac gauge doesn’t indicate one, idle quality is unaffected, and it happens when manifold vac should be fairly low.
