Reciprocating engines are still frequently used in aviation especially in applications such as recreation planes, taxi-planes, fire extinguishing aircraft and generally applications that do not require a high power density. For such applications they have a significant advantage against turbine engines as far as purchase and maintenance cost is concerned. The proper and efficient operation of these engines in aviation applications is critical and therefore techniques that are used to determine engine condition and to detect potential faults are extremely important. The performance of these engines depends strongly on the condition of the ignition system and the quality of the supplied mixture. For this reason in the present work it is examined the effect of mixture AFR on the combustion mechanism and engine performance using an existing diagnostic methodology for spark ignited engines developed by the present research group. The investigation is conducted on a radial, spark-ignited reciprocating engine used on the CL-215 fire extinguishing aircraft. The diagnostic technique is used to investigate the effect of AFR on the main combustion and performance characteristics of the engine and specifically brake power output, rate of heat release, cumulative heat release, peak firing pressure, ignition and injection timing and duration of combustion. Furthermore the diagnostic technique is used to derive information for spark advance, spark duration, compression condition etc. The diagnostic technique is based on a thermodynamic two-zone combustion model for spark ignited engines. To examine the effect of AFR on the combustion mechanism a detailed experimental investigation was conducted on an engine (radial, supercharged, air-cooled, eighteen-cylinders) mounted on a test bench. The measurement procedure involved measurements at various operating conditions (load and speed) and various AFR values. During the experimental investigation beyond the conventional test bench measurements, measurements were taken using a fast data acquisition system of cylinder pressure and the electric signal of both spark plugs. Engine diagnosis is established by processing of these measured data. From the results of the diagnosis procedure it is revealed that the diagnosis method provides detailed information for the operating condition of the engine and the values of parameters that cannot be measured on the field. The diagnosis results reveal that the proposed technique can determine the effect of AFR ratio on the combustion mechanism adequately and thus it can be used during engine testing to determine the optimum AFR ratio in combination with the remaining engine settings and mainly spark advance. The results obtained are positive and reveal that the proposed diagnostic technique can be easily applied on any type of spark-ignited engine and especially on aircraft piston engines (i.e. aviation applications), where the accurate estimation of the engine condition and settings is extremely important.