Despite the known benefits of direct injection (DI) spark ignition (SI) engines, port fuel injection (PFI) remains a highly relevant injection concept, especially for cost-sensitive market segments. Since particulate number (PN) emissions limits can be expected also for PFI SI engines in future emission legislations, it is necessary to understand the soot formation mechanisms and possible countermeasures. Several experimental studies demonstrated an advantage for PFI SI engines in terms of PN emissions compared to DI. In this paper an extended focus on higher engine loads for future test cycles or real driving emissions testing (RDE) is applied. The combination of operating parameter studies and optical analysis by high-speed video endoscopy on a four-cylinder turbocharged SI engine allows for a profound understanding of relevant soot formation mechanisms. For selected operating points, engine operating parameters such as injection timing, inclination of a charge motion flap, and engine coolant temperature were varied. Furthermore, the impact of two different spray layouts on the mixture formation was evaluated. Parameter sets showing significant reduction of PN emissions were subsequently analyzed using high-speed video endoscopy. Optical access to both the intake port as well as the combustion chamber allows visualization of fuel transport mechanisms leading to diffusion flames and soot emissions respectively. In summary, this study shows that port fuel injection at high engine loads can lead to significant PN emissions. Three locations within the combustion chamber could be identified as sources for diffusion flames leading to particulate emissions. The governing parameters allowing substantial reduction of PN emissions at these locations were found to be the injection timing and the charge motion.