The ratio is about 15:1 fuel to air.

The air fuel ratio of the petrol engine is controlled by Carburetor

Air/Fuel Ratio

It is when the "correct" air to fuel ratio is used to completely burn the fuel in question in an internal combustion engine.

If exactly the correct amount of air is used this is called the stoichiometric mix.

Air to fuel ratio is the ratio between the weight of air to the weight of fuel, for example: 14.7 pounds of air to 1 pound of fuel would be a air:fuel ratio of 14.7:1 This particular example is called the stoichiometric ratio for gasoline...it's ideal ratio in order to efficiently use all the fuel. This ratio varies for different fuels....for example, ethanol fuels have a stoichiometric air:fuel ratio of 13.85:1. In actuality, a gasoline car gets it's best fuel mileage when the fuel is reduced slightly and produces it's best power when the ratio is a little heavier on the fuel side.