Too much fuel in the fuel-air mixture is referred to as too “rich”; not enough fuel is too “lean”. The “mixture” is normally controlled by adjustable screws on an automotive carburetor, or a pilot-operated lever on piston-engined aircraft (since mixture is air density (altitude) dependent). The (stoichiometric) air to petrol ratio is 14.6:1, meaning that for each weight unit of petrol, 14.6 units of air will be burned. In theory this the most efficient regarding the power/consumption ratio. But as combustion chambers in use on engines are not able to allow complete combustion of the gasoline with stoichiometric mixture, a mix around 18:1 will give better results regarding fuel economy and pollution, the excess air allowing more complete combustion. A richer mixture around 11:1 will deliver more power as the excess fuel will cool the cylinders and pistons but the price to pay is increased consumption and environmental pollution.
Carburetor adjustment can be checked by measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust gases. The mixture can also be judged by the state and color of the spark plugs: black, dry sooty plugs indicate a too rich mixture, white to light gray deposits on the plugs indicate a lean mixture. The correct color should be a brownish gray. See also reading spark plugs. In the early 1980s, many American-market vehicles used special “feedback” carburetors that could change the base mixture in response to signals from an exhaust gas Oxygen sensor. These were mainly used to save costs (since they worked well enough to meet 1980s emissions requirements and were based on existing carburetor designs), but eventually disappeared as falling hardware prices and tighter emissions standards made fuel injection a standard item.
There are persistent rumours that appear to extend into the realm of urban legend or even into conspiracy theory of extremely efficient carburetors. However, there may be some basis for these rumors or claims. A catalytic carburetor mixes fuel fumes with water and air in the presence of heated catalysts such as nickel or platinum. The fuel would break down into methane, alcohols, and other lighter-weight fuels. The original catalytic carburetor was introduced to permit farmers to run tractors from modified and enriched kerosene. The U.S. Army used catalytic carburetors in World War II in the North African desert campaign, it has been said, to achieve substantial logistic surprise and thus tactical and strategic advantage against the Germans.
However, it is known that less than two years after commercial introduction of the first catalytic carburetor, in 1932, tetraethyl lead was introduced as an additive to raise gasoline’s resistance to spontaneous combustion, thereby permitting the use of higher compression ratios. Also in that time, the price differential between a thermal calorie of gasoline and kerosene was ended. Tetraethyl lead had the effect of poisoning catalytic carburetors. Many modern gasolines appear to have additives for “cleaning” which perform the same effect by producing varnishes or gums in the presence of water, which of course, is not a recommended use. Gasoline/petrol is an impure mixture of linear heptane and octane and other miscellaneous light alkanes. Commercial gasolines usually contain additives to clean engines, artificially lower evaporation points, and (conjecturally) poison catalytic carburetors (an effect that is certainly real, but might be accidental).
Famed NASCAR mechanic Smokey Yunick spent many years working on a high fuel economy “vapor carburetor”. The detailed operation is not widely disseminated, but the general principle is to heat the fuel with waste engine heat to enhance vaporization and improve the fuel’s combustion characteristics. This was reasonably effective compared to normal carburetors of the time, but had implementation difficulties. During the years he was developing it, the average production engine moved from centrally located carburetors to electronic fuel injection, wherein the fuel is delivered right to the intake port. This dramatically reduces fuel condensation and puddling in the intake manifold and runners, making Smokey’s design a solution to a problem which no longer existed so it was never commercially developed.
Some great references can also be found in the Books Reference page: