In the next incoming future the necessity of reducing the raw emissions leads to the challenge of an increment of the thermal engine efficiency. In particular it is necessary to increase the engine efficiency not only at full load but also at partial load conditions. In the open literature very few technical papers are available on the partial load conditions analysis. In the present paper the analysis of the effect of the throttle valve rotational direction on the mixture formation is analyzed. The engine was a PFI 4-valves motorcycle engine. The throttle valve opening angle was 17.2°, which lays between the very partial load and the partial load condition. The CFD code adopted for the analysis was the FIRE AVL code v. 2013.2. The exhaust, intake and compression phases till TDC were simulated: inlet/outlet boundary conditions from 1D simulations were imposed. The injection system operation was experimentally investigated in terms of spray shape and drop sizing and velocity for a proper tuning of the numerical model. The injection process was modelled and the final results in terms of mixture composition and turbulence level at the ignition time were investigated. The aim of the paper was to deeply analyze the dynamic effect of the throttle valve position on the engine behavior. The wallfilm effect on the effective mixture formation process was considered by means a new methodological approach.The wallfilm thickness and its dynamics affect the final mixture formation process and the level of the mixture index at the ignition time close to the spark plug. It is also necessary to consider that, even though the CFD simulations were RANS simulations, it could take some days for reaching the converged wallfilm thickness, even 20 or more engine cycles at full load conditions could be necessary. The research group proposed a new methodological approach for facing this problem within a computational time compatible with industrial applications too.