There are a few reasons why a motorcycle's engine may need to be revved higher before starting:
Cold engine: Cold engines, especially in colder weather conditions, require a richer air-fuel mixture for efficient combustion. By revving the engine higher before starting, it increases the airflow and fuel supply, helping to ensure sufficient fuel vaporization and ignition. This can aid in starting the engine smoothly, especially when the engine is cold and the fuel may not vaporize as easily.
Carbureted engines: Some older motorcycles or smaller displacement motorcycles may still use carburetors instead of fuel injection systems. Carburetors rely on a precise air-fuel mixture for combustion. By revving the engine, it increases the air velocity through the carburetor, assisting in drawing fuel into the cylinders and establishing the proper mixture ratio.
Choke or enrichment circuit: Motorcycles equipped with a choke or enrichment circuit often require higher revs before starting. The choke restricts the air supply, creating a richer fuel mixture. Revving the engine higher helps to draw more fuel through the enrichment circuit, providing the necessary fuel for a successful start.
Low battery voltage: In motorcycles with electric starters, a low battery voltage can affect the cranking speed and the ability to start the engine. Revving the engine higher before starting can help generate more electrical power from the alternator or charging system, potentially providing a stronger spark for ignition.
It's important to note that excessive or prolonged high-revving before starting can put unnecessary stress on the engine components. Therefore, it is generally recommended to follow the manufacturer's guidelines for starting procedures and allow the engine to warm up properly after starting to ensure optimal performance and longevity.