It does sound too lean if you have to choke it hot. Look for vacuum leaks, if a siamese manifold possibly it is burned through internally.
You say it won't idle down to 400, does it die or will it just not go down even with the idle stop backed off?
If it won't idle without dying, that can be low compression, or idle circuit failure. Idle circuit failure is caused from clogged passages or the bowl gasket not sealing where the fuel is drawn up. Also be sure the throttle plate will close light tight with the stop screw backed off. If not the plate may be in backwards or not centered.
Look up a fuel circuit diagram online for that carb, you can trace the idle circuit and get a better idea of the flow, mixing, and adjustment function. Remember, you are adjusting air bleed, not fuel flow. Backing the screw out leans it. The idle screw will have no effect above about 500 RPM.
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Today's Featured Article - Memories of an IH Super A When I was ? up to 10, I worked on my Papaw's farm in Greeneville, TN every summer. As I grew older (7), it was the thrill of my day to ride or drive on the tractor. My Papaw had a 1954 IH Super A that he bought to replace a Cub. My Papaw raised "baccer" (tobacco) and corn with the Super A, but the fondest memory was of the sawmill. He owned a small sawmill for sawing "baccer" sticks. The Super A was the powerplant. When I was old enough (7 or 8), I would get up early and be dressed to
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