Don't get too whacked out over the carb, I've seen these work, and other just behaving as a paperweight for no known reason at all. Like straight out dead from factory. A simple mistake like forgetting to drill a hole in the cast could totally throw off a carburetor. Even other microscopic flaws (cracked cast or a bubble trapped in the wrong place) can just make it impossible to properly tune one in. It just happens and since there's little to no chance that they hook these up to motors and test them before they leave the factory then there just has to be a few defects around.
I've had units just refuse to run a motor at all, swap it out for an identical (brand and settings) carb and it starts right up. The only explanation is that the carb is junked. Some spots on a carb can be very difficult and even impossible to reach with a cleaner unless you physically removed plugs from the cast to access the little side ports that do the magic.
My current carb uses 2 jets, and the idle jet has its flow of fuel go in the idle jet but has a small hole that takes it off to the side where it meets the idle air mixture screw and pathway for the fuel to the intake. If somehow that would get clogged or be left with flakes of cast in it it would not hold an idle, and I can see cleaning that particular path in the carb as being difficult because of the location/shape.
Yeah uh... If you can faithfully eliminate all the other problems then look at the possibility that the carb is physically imperfect to the point where it can't perform the job is meant for.