12-yr old 395CAV Series F gas furnace in my recently bought house produces soot deposits wherever there s air movement, on air return register, kitchen exhaust fan and light cover, on spiderwebs on houseplants etc. It's lamp black. Have put in new store-bought air filter at the place where the former owners left a dirty one. Original filter was factory provided or so the manual says. Have had gas heat before and never saw such soot formation. I'm sure something is wrong - maybe the gas to air ratio is too high?
Former owners had a C monoxide detector and the hoouse was inspected before I bought it and no problem was seen then, less than 2 months ago. The deposits are much older.

Any ideas before I call the vendor who is going to try and sell me a new furnace?