This paper investigates abnormal combustion during the cranking phase of spark-ignition small engines, specifically the occurrence of backfire at the release of the starter motor during kickback. The research focusses on the influence of fuel composition, mainly in terms of ethanol percentage, on backfire occurrence. Interest in this abnormal combustion is growing due to the increased use of fuels with different chemical-physical properties with respect to gasoline. Moreover, this issue will become even more topical due to the implementation of simple control and fuel supply systems on low cost-engines, which are widely used in developing countries.Experimentation was carried out in an optically accessible engine derived from a 4-stroke spark ignition engine for two-wheel vehicles. The test bench was instrumented and adapted in order to simulate the engine conditions that lead to anomalous ignition in the intake duct (backfire) during the reverse rotation of the engine (kickback). Two different test procedures were developed with the aim of promoting the ignition at the intake.Major engine parameters were measured, such as the in-cylinder pressure, the pressure at the exhaust and at the intake; in order to characterize the engine conditions and to monitor the ignition in the intake manifold. Furthermore, an optical investigation of the combustion chamber was performed using high spatial and temporal resolution measurements.Engine and optical data were correlated and the analysis allowed characterization of backfire during kick-back, enabling identification of the conditions that make backfire more probable and to observe the influence of fuel composition.