Black smoke = incomplete combustion. Check air filter first as it's easy to do. Excessive load will have the same effect. Does the prop have lots of growth on it? Is something binding on the shaft? (check by turning it by hand. it should be easy to do (with the
engine in neutral!)). After that you're looking at
fuel issues. Governor,
fuel pumps, fuel quality, blocked filter, sludge in the fuel, worn injectors etc.
disclaimer: i'm not a mechanic!