Too much engine oil. Check and adjust the quantity.
Too low viscosity of engine oil. Check and change.
Improper fuel. Flush & change oil
Improper injection timing. See dealer
Engine oil burning or detonation. See dealer
Overload. Reduce the load
Clogged air cleaner. Check and change.
Improper fuel. Check and change.
Excessive fuel injection. See dealer
Improper function of fuel injection