oil furnace with a lot of soot in pipes

i can upload a pic if it helps. this furnace hasn't been cleaned in at least 12 yrs. i'm hoping that's the cause of all the soot, and why it won't fire. i've changed the nozzle, filter, bled the air. to remove what i'm guessing are the heat exchangers looks very difficult. is it necessary to open/clean heat exchangers right above the burner, or is it too hot for soot to accumulate so close? does the soot start to accumulate at the chimney then build up towards the furnace? from the back end, i can see buildup on the sides, but they are not completely blocked. i had a tech come out, and i'm guessing he didn't want to clean the thing. this is a 1993 Carrier(Ducane) model # 58 BLA075-12