Bought my first mopar 73’ Duster and the bottom of the air filter pan is black (right above the carb). Any tips on how the 4bbl should be adjusted

The thing is, if it's a 4bbl, it may have been re-cammed with a hotter cam, and therefore tune up specs don't apply. In fact a 73 was so "smoggy" (emissions restricted) that if the stock distributor advance is still in place, you can improve upon that greatly by installing a re-curved or aftermarket performance distributor. The "quick" way to this is an outfit called FBO ignition which offers a kit "limiter advance plate."

Generally tho, since you have a new-to-you car, you don't know the history, I would do some general diagnostics and tune up. Run a compression check/ leak down test to evaluate compression. Look at the plugs. Are they too lean, what is their heat range, and any signs of oil use. Does the car puff any smoke when revving, when "letting off" or when accelerating? How does the engine sound, generally, any noises that don't belong?

Then endeaver to check the timing, and try to graph the advance curve. Check initial timing with vacuum hose disconnected, Mark balancer if needed to determine higher RPM mechanical advance, and finally reconnect vacuum advance hose and wind it up and see what "high cruise" full advance including vacuum gives you. Note these numbers.

ADJUSTING CARB You can ONLY adjust carb idle speed an mixture AFTER setting timing, and AFTER making sure that the ignition system, cap, rotor, wires, plugs, etc are in good shape.

And the engine MUST be fully warmed, so if it runs "cold" you need to find the cause FIRST

With this done, and the engine warm, adjust the idle speed screw, over on the driver side carb linkage, to somewhere close to where you want it to idle. With a tach or vacuum gauge, adjust the screws first in, then out, to find highest RPM, highest vacuum. Go back and forth a couple of times. Then once you have found the "peak" just touch the screws so the idle does not quite drop, as you screw the adjusters in a tiny bit leaner.

Next, if this is an automatic. check the idle speed in drive. Be CARFUL, use a buddy to hold the brake, or block the tires. You may have to change the idle speed/ mixture to get this where you want.