While burning hydrocarbon fuels, if we see a yellow flame with lots of black smoke, it means that the fuel is :

The appearance of a yellow, luminous flame accompanied by black smoke during the burning of hydrocarbon fuels indicates that the fuel is made of unsaturated hydrocarbons. Saturated hydrocarbons, such as alkanes, typically burn with a clean, blue flame because they have a lower carbon-to-hydrogen ratio, allowing for complete oxidation in air [1]. In contrast, unsaturated hydrocarbons like alkenes and alkynes contain a higher percentage of carbon, which does not oxidize completely in the presence of atmospheric oxygen [1]. This incomplete combustion results in the formation of unburnt carbon particles, commonly known as soot or black carbon, which glow yellow due to heat and produce black smoke [3]. While limiting the air supply can cause saturated hydrocarbons to produce a sooty flame, the characteristic behavior of unsaturated compounds in normal air is the production of a yellow, smoky flame [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.4 > p. 69
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Why do substances burn with or without a flame? > p. 70
3. [3] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 17: Climate Change > 17.3.6. Black Carbon > p. 258