a.Poor combustion due to faulty injection system
b. Low compression
c. Insufficient scavenging air
d. Fouled exhaust system
e. Broken piston rings
f. Ineffective lube oil seal
g. After burning, bad fuel and other fuel assisted faults
h. Faulty cylinder lubrication
i. During starting, it is inevitable as the engine maker sets the starting fuel index to a fixed value. At this index there is insufficient air, low piston speed, cold combustion chamber, low fuel injection pressure etc., each compounding the cause for black smoke.