The metal tube from the oil fill tube is indeed for crankcase ventilation. It should connect with a port on the air cleaner so fumes are directed into the carburetor.
The large spring loaded screw adjusts the amount of additional air that blends into the air/fuel mixture drawn up the low speed jet (aka idle tube). If there is little change in idle speed when adjusted from closed to 1 1/2 turns open there is a problem in that circuit. The majority of used carburetors have the screw closed to only slightly open, primarily due to wear on the throttle shaft and carburetor body. As long as a reasonably smooth idle is found the extra expense to replace the throttle shaft and bush the carburetor body is not justified.
Because of the octane rating of modern gas 85+ it burns slower than the engine is designed to use. That was in the 50 - 60 range which burned much faster. The result is incomplete combustion or carbon formation. That can be reduced or eliminated by increasing the timing to ~ 18 deg. btdc and increasing the gap on the spark plugs from 0.025" to 0.040". The hotter spark at the plugs and timing increase recovers the power that higher octane gas causes.
Your 1930 distributor has a mechanical advance in addition to the spark retard cable. With the mechanical advance working properly there is little reason to even touch the "spark" knob on the dash. It is there to be able to retard the spark if hand cranking the engine. Chevrolets have advanced features compared to Model A Fords.
The hub caps use spring loaded retention tabs. If the springs are weak or tabs worn the caps can be loose on the wheels. Some have used electrical tape or other materials to shim out the space and tighten loose caps.
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