Why Does a Motorcycle Make Abnormal Noises at Idle Speed?

[DiReagan]

Motorcycle abnormal noise at idle speed is caused by piston slap. Below are the relevant details: 1. The cause of piston slap noise is usually an excessive clearance between the piston and the cylinder. During the engine's power stroke, the piston is subjected to high-pressure gas forces, causing it to swing and knock against the cylinder wall, producing noise. Additionally, piston reverse ovality, bent or twisted connecting rods, excessive tightness in the piston pin and connecting rod bushing (bearing), or big-end bearing can also lead to piston slap. 2. Characteristics of big-end bearing noise: During sudden acceleration (from idle to medium speed), there is a distinct continuous knocking sound (rolling bearings may also produce internal engine noise). This is the main characteristic of big-end bearing noise. When the big-end bearing is severely worn (or loose), the noise is noticeable even at idle speed and worsens under increased load.