Meanz

Hi forum, i have a 1997 c180 w202. It had excellent fuel economy when i first had the car, but now it is very very bad. A full tank would give me 300 kms if I'm lucky where before i used to get almost 700 kms. Also everytime i accelerate i see black smoke coming from the exhaust(which i assume is unburnt fuel). I took out the spark plugs and found them black. I changed the air flow meter with no change. Anyone have ideas what could be causing this massive fuel consumption?
Thanks