Refers to carburetor setting. When an engine is run too lean it will overheat, causing damage, and likely an in flight engine failure. Tuning a carburetor is best accomplished by starting rich, and working gradually to the condition which produces maximum power, while allowing a small amount of unburned fuel mixture to lubricate and cool the engine.
carburetor setting are called lean when the mixture of fuel and air are biased toward more air than would be used in the standard mixture.
A condition where the air-fuel mixture is not the correct ratio (not enough fuel relative to the amount of air). High HC and/or high NOx emissions are symptoms of a lean air fuel ratio.
When the air - fuel mixture is too rich, the driver may adjust the needles to lean out the mixture, i.e. increase the air to fuel ratio. If the engine runs too lean, it may get too hot which can severly damage the engine. The piston may get stuck inside of the cylinder.
When the needles on the carburetor are set such that the engine is not getting enough fuel, this is referred to as running lean. Running too lean may help fuel economy and appear to make the vehicle run better; however, it is also a sure way to shorten the life of the engine.