It's when there's a little less fuel for the amount of air than the ideal condition.
basicly its changing the mixture of fuel/air to improve performance
A lean fuel mixture occurs when there is an excess of air relative to fuel in the combustion process. This can be caused by factors such as a malfunctioning fuel injector, a vacuum leak, or improper tuning of the engine. Additionally, environmental conditions like high altitude can also lead to a lean mixture, as there is less oxygen in the air. When a lean mixture is present, it can result in higher combustion temperatures and potential engine damage if not addressed.
install a cold air intake. more air with same amount of fuel being injected will lean out the fuel/air mixture. plus you'll get more power
turn clockwise or inward to lean the mixture.
The ideal fuel/air mixture or ratio for most internal combustion gasoline engines is 14.7 to 1. Meaning 14.7 parts air to one part gasoline. Any ratio below this is considered a rich mixture or too much fuel. Any ratio above this is considered a lean mixture or too much air.
Dry is good, but white is bad! It means that the engine is running lean, which will result in the engine running too hot and can cause serious engine damage (even seizure). Below is a guide to reading the spark plug color. Black = Rich. The fuel/air mixture is too rich (too much fuel, not enough air). Tan/Brown = Normal. The fuel/air mixture is correct. White = Lean. The fuel/air mixture is too lean (too much air, not enough fuel).
It means the amount of air in the fuel/air mixture going into the cylinders it too high.
51** Fuel System Lean A lean air/fuel mixture has been indicated by an abnormally rich correction factor
LEAN
The O2 sensor is normally in your air intake, or throttle body. It moniters the air flow into the motor that mixes with the fuel. Your CPU controls how much air goes into the engine. A rich fuel mixture robs horsepower while a lean fuel mixture robs reliability. All in all, the Fuel/oxygen sensor measures rich/lean and adjusts the mixturea ccordingly
A lean air-fuel mixture refers to a ratio in which there is more air than fuel, typically having an air-to-fuel ratio greater than 14.7:1 by weight for gasoline engines. This means that there is less fuel relative to the amount of air, which can lead to more complete combustion and improved fuel efficiency, but may also cause higher combustion temperatures and potential engine knock if excessively lean.
To lean the carburetor, you should turn the mixture screw in (clockwise). This reduces the amount of fuel entering the mixture, resulting in a leaner fuel-to-air ratio. Conversely, turning the screw out (counterclockwise) will enrich the mixture by allowing more fuel to flow. Always make adjustments gradually and monitor engine performance.