Take the freewheel off the hub. Flush with WD-40 or other penetrant (KROIL, etc) and spin until it frees up and spins easily and smoothly. Don't go lightly - hose it down repeatedly and liberally while spinning.
Follow up with light oil of your choice - motor oil, gun oil, etc.
Alternately, you can soak it in a dish of kerosene and spin the gunk out. Lube as above.
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"Love is not the dying moan of a distant violin, it’s the triumphant twang of a bedspring."
S. J. Perelman