The effects of several operating parameters on dual fuel combustion at light load were investigated by means of direct endoscopic observation of the process. Therefore, an intense experimental campaign was performed on a single cylinder diesel common rail research engine, converted to operate in dual fuel mode and equipped with optical accesses and variable intake configuration. Three bulk flow structures of the charge were induced inside the cylinder by activating/deactivating the two different inlet valves of the engine (i.e. swirl and tumble). Methane was injected into the inlet manifold at different pressure levels and varying the injector position. In order to obtain a stratified-like air-methane mixture, the injector was mounted very close to the inlet valve, while, to obtain a homogeneous-like one, methane was injected more upstream. By combining the different positions of the methane injector and the three possible bulk flow structures, seven different engine inlet setup were tested. Moreover, pressure and quantity of the diesel pilot injection were varied. For each acquired combustion image, the luminance plane was extracted and a luminance value, averaged over the whole frame, was calculated in order to obtain an indicator of the combustion intensity. These crank angle-based luminance curves were compared while the total integral and the peak values were calculated. From the analysis of the luminance curves it can be observed that the in-cylinder bulk flow associated with the swirl port is characterized by a more rapid development of the combustion. Especially for certain combinations of the engine operating parameters, higher peaks of luminance values can be noticed while the luminance curves fall to zero earlier with respect to the other inlet configurations. Concerning the methane injector position, some noticeable effects on the intensity and distribution of the flame during dual fuel combustion were observed. Depending on the bulk flow structure induced inside the cylinder, methane injector position can induce a certain degree of stratification of the in-cylinder charge, capable to enhance dual fuel combustion at low loads.