This paper presents an air-fuel ratio control of a spark ignition (SI) engine. The control strategy depends on the evaluation of the model parameters such as fuel puddle parameters, time constant and delay time values using fuzzy logic. These values in turn are used in the air-fuel ratio control that implements self-tuning regulator (STR) system. The effectiveness of the proposed design is demonstrated through simulation for various throttle transients. The results obtained show that the air-fuel ratio is maintained within 0.5% of the commanded stoichiometric value.