300SDL 1987: Can you please help me identify what the hose with the filter needs to be connected to?

My daughter and I bought a run down SDL. The engine starts well even in our very cold winter weather now, but it does not have power and is very slow with a sizeable amount of black smoke coming from the exhaust when I push the accelerator all the way down. I noticed that there was one hose from intake manifold to the valve mounted to the fire wall had fallen off. The previous owner had overfilled the engine with 2 extra quarts of oil. I corrected both and the engine is still not developing an awful lot of power though it sounds and run already a lot better.

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