What are the causes of valve leakage?

[SanJocelyn]

Here are the causes of valve leakage: 1. Wear, burning, spots or depressions, and carbon deposits on the working slope of the valve and valve seat ring. 2. Excessive clearance between the valve stem and valve guide, causing the valve stem to wobble or bend, or the valve head to tilt, resulting in incomplete valve closure. 3. Weakened or lost elasticity of the valve spring or a broken spring can also prevent the valve from tightly fitting with the valve seat. 4. Small valve clearance causes the valve stem to expand when heated, and the tappet and rocker arm push the valve open, preventing it from fully closing and leading to leakage. 5. Prolonged operation of the engine at high speed and temperature weakens the valve spring tension, preventing the valve head from pressing tightly against the valve seat. 6. Failure of the decompression mechanism, causing the valve to close incompletely.