What is the reason why there is unburned gasoline in the exhaust of underpowered cars?

Tianjin Shengwei Technology answers for you:

What color is the exhaust gas? Is it black? If it is black, it means that the mixture is too rich to burn well, the unburned gasoline forms black carbon particles, which form black smoke after being discharged, or the spark plug fails, resulting in too small ignition advance angle and incomplete combustion.

If white smoke is emitted, it is because the individual cylinders of the ignition system are not ignited, and the mixed gas entering the cylinders is directly discharged without burning, and the gasoline vapor condenses into white smoke again.