The ideal idle RPM (revolutions per minute) for an engine can vary depending on several factors, including the engine type and its intended use. Generally, the recommended idle RPM range is specified by the manufacturer and can be found in the owner's manual or service documentation for the specific engine model.
Different engine types, such as two-stroke and four-stroke engines, may have different recommended idle RPM ranges. Additionally, the intended use of the engine can also influence the ideal idle RPM. For example, a racing motorcycle may have a higher idle RPM compared to a regular city bike, which is designed for commuting in urban areas.
Since you mentioned a quarter-liter city bike, I assume you are referring to a motorcycle with an engine displacement of around 250cc. The recommended idle RPM for such a motorcycle can vary depending on the specific model and manufacturer. It is best to consult the owner's manual or contact the manufacturer directly to obtain the accurate and specific information regarding the recommended idle RPM for your particular quarter-liter city bike.