Note: this article is not meant to suggest or imply causes – only to determine the nature of the symptom.

Lean:

Applicable to all throttle openings

• The engine runs better as it warms up.
• The engine runs better when the ambient temperature is higher.
• The engine runs better at higher altitude.
• Slightly opening the cold start circuit (or closing the choke) reduces the symptom (see Types of Motorcycle Carburetor Cold Start Devices).

Applicable to small throttle openings

• Opening the fuel screw reduces the symptom.

Applicable to large throttle openings

• The engine runs better when the airbox opening (or filter) is partially restricted.

Rich:

Applicable to all throttle openings

• The engine runs better when it is not up to its operating temperature.
• The engine runs better when the ambient temperature is lower.
• The engine runs better at lower altitude.

Applicable to small throttle openings

• The engine does not require the cold start circuit to be opened very much or at all (or the choke to be closed) for it to be able to start from cold (see Types of Motorcycle Carburetor Cold Start Devices).
• Partially or fully closing the fuel screw reduces the symptom.

Applicable to large throttle openings

• The engine runs better when the intake is opened (i.e. airbox lid or air filter removed).