An RC car engine's carburetor (or any carburetor, really) allows a car's fuel to mix with the air (specifically oxygen), preparing it for ignition. The carb controls the ratio of fuel to air, thereby controlling the performance of the engine. The air:fuel ratio dictates the engine's performance.
Most RC car carburetors (carbs) have a low-speed needle and a high-speed needle. Some also have a "third needle" or mixture control valve. They also have an idle screw which is used to set the amount of air the engine gets during idle (the idle gap.)
The needles allow you to control the fuel mixture at different parts of the engine's range - the low-speed needle controls the fuel mixture at idle and low throttle, the mixture control valve controls the mid-range, and the high-speed needle sets the mixture at full throttle.
Rich mixture / higher fuel-to-air ratio:
- More lubrication in engine
- Engine runs cooler
- Lower rpms
Lean mixture / lower fuel-to-air ratio:
- Nitro is fully burned, so less lubrication in engine
- Runs hotter
- More rpms
You may need to tune the engine to be more lean if:
- Temperature increases
- Altitude increases
- Humidity lower
And vice versa - you may need to run the engine more rich if any of these reverse.