The usual culprit is something that built a nest (spiders, etc.) in the burner tube or the flame tube that runs through the tank. The black smoke sounds like the flame isn't burning clean in the burner tube. Try a few blasts of compressed air through the burner and the flame tube then watch, with the outer door open, and see if it changes and if there is a blue flame with little or no yellow going into the flame tube through the tank. The exhaust vent at the top left should have just hot air comming out.
Gene & DW Ginny
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