What is a burnt valve in a motor of a car?

a burnt valve looks like a wide crack in a valves' head. when a motor runs too lean, the cylinders heat up too much, causing the head of the valve to heat up faster than it can dissipate the heat, causing the valves' head to crack. once the valves' head starts cracking or melting away, you lose compression in that cylinder, and the engin will vibrate and it may not idle at all.