Why is the engine of a scooter motorcycle so loud?

[StephanieLee]

Engine abnormal noise manifestations and their causes are introduced as follows: Backfire sound: The noise is loud but not continuous. It manifests as backfiring from the exhaust pipe and overheating during engine operation. The main cause of this phenomenon is an excessively small ignition advance angle, where the combustible mixture does not fully burn in the cylinder. When the unburned mixture encounters heat in the exhaust pipe and spontaneously ignites or is ignited by sparks, it combusts and explodes, resulting in the backfire sound. Knocking sound: When the engine is running, a sound similar to a wooden hammer striking can be heard by placing a "stethoscope" against the cylinder head, which is more noticeable at low speeds or when changing throttle. This knocking sound is mainly caused by detonation or pre-ignition due to an excessively large ignition advance angle, excessive carbon deposits in the combustion chamber, or an overly high compression ratio. Misfire sound: This manifests as occasional one or two misfire sounds in the otherwise uniform and regular exhaust noise during engine operation. This phenomenon is primarily caused by burned breaker contacts or excessively small contact gaps.