The main function of the carburetor is to create a proper mixture of fuel and air. Once this mixture is achieved, it is sent to the engine to be combusted. The carburetor can be adjusted to control the mixture ratio of gas to air that is fed into the engine. If you increase the amount of fuel in the mixture, you are making the mixture "rich" and alternatively, if you decrease the amount of fuel in the mixture, you are making the mixture "lean".
An engine's performance is dependent upon this fuel mixture and is regulated by the carburetor, which, in turn, is regulated by a high speed, low speed, and idle speed setting.
Important Note: Make sure to completely break-in your nitro engine prior to tuning the engine for maximum performance. See Breaking in Your New Nitro Vehicle.
Important Note: Turn the mixture needles clockwise to create a “lean” mixture and counterclockwise to create a “rich” mixture. A lean fuel mixture will increase engine power, but be wary.
Caution: Never run an engine too lean, because it will cut out or begin to stall. This could cause severe engine damage.
In order to tune your RC for performance, start with a rich fuel mixture and slowly move towards a lean mixture until the ideal mixture ratio is found.
Caution: Never EVER tune from a lean to a rich mixture. There should always be a light stream of smoke coming from the exhaust.
You can tell an engine is running rich by observing any of the following:
Leaning the high speed fuel mixture increases performance
Unburned fuel spraying from the exhaust tip