System for controlling the carburetion of an internal combustion engine comprising the following activities: a) starting the engine; b) determining the factor λo using any known method; c) measuring the ionization current cio; d) modifying—from outside—the factor λ by a predefined amount equivalent to Δλ bringing it to the value λ1=λo+Δλ; e) repeating the measurement of the ionization current ci1; f) calculating the value of the difference of the ionization current Aλci1 equivalent to cio−Ci1; g) repeating operations d. and e. until the difference between the last value of the ionization current o,η and the second last value cin-1 is smaller than Δci; where Δci is a predefined value which is assumed to have no influence with respect to the carburetion.