Carburettors are prone to accumulation of gasoline residue which affects their function. If gasoline evaporates, it may leave deposits in the form of gum, similar to tree sap. If the gum dries hard, it is said to be "varnish" because it cannot be wiped clean without a powerful solvent. The composition of carburettor cleaner is specifically designed to clean residue in carburettors without damaging gaskets, rubber or plastic parts.
There are as many compositions or formulas for carburettor cleaners as there are brands. The primary ingredients are organic solvents. Fuels that carburettors atomise are organic solvents themselves, such as benzene, ethanol and methanol. Carburettor cleaners use some of these very solvents, without the fuel additives that cause gumming and lacquering. Different compositions are designed for slightly different applications. For example, some compositions my be better solvents but have drawbacks including a greater hazard damaging plastic parts and gaskets or causing respiratory problems for some users.
Organic solvents are also frequently used in carburettor cleaners. A very common organic solvent used in carburettor cleaner composition is acetone. It is an extremely effective solvent. Just as it works as fingernail remover, acetone chemically breaks down the gum or lacquer into a liquid and washes it away. Toluene, methyl ethyl ketones and 2-butoxyethanol are also very common organic solvents.
Carburettor cleaner includes oils. The oils leave a protective residue on the parts after they've been cleaned, very similar to gun oil, 3-in-1 oil or sewing machine oil. Other parts cleaners, such as brake cleaner, do not use such oils specifically to prevent any post-cleaning residue or lubrication.