Incomplete combustion of a hydrocarbon:

A reaction in which:

Incomplete combustion occurs when there is not an excess of oxygen.

What happens to the carbon atoms from the hydrocarbon in a complete combustion reaction?

Problem with incomplete combustion:

Carbon monoxide is toxic - It combines with haemoglobin in red blood cells and prevents oxygen from being delivered to bodily tissues
Soot can block pipes that carry waste gases away from an appliance
Soot causes breathing problems if it collects in the lungs