In complete combustion, there is a sufficient supply of oxygen which is able to react with the burning hydrocarbon allowing the reaction products H2O and CO2 to be formed.
In incomplete combustion there is a lack of oxygen so when heat is applied carbon monoxide is released.
For example: C2H6 + 3 1/2O2 -> 2CO2 + 3H2O C2H6 + 3O2 -> CO + CO2 + 3H2O