Carbon is often deposited in the exhaust-pipe of cars because of the

The carbon deposits in the exhaust pipe of cars are caused by the incomplete combustion of petrol. When petrol is burned, it reacts with oxygen to produce carbon dioxide and water vapor. However, if there is not enough oxygen, the petrol can only partially burn, leading to the formation of carbon particles (soot) that get carried through the exhaust system and eventually settle in the exhaust pipe. The presence of additives in petrol or contamination with diesel would not directly cause carbon deposits in the exhaust pipe.