Pre-chamber combustion (PCC) allows an extension on the lean limit of an internal combustion engine (ICE). This combustion mode provides lower NOx emissions and shorter combustion durations that lead to a higher indicated efficiency. In the present work, a narrow throat pre-chamber was tested, which has a unique nozzle area distribution in two rows of six nozzle holes each. Tests were carried out in a modified heavy-duty engine for optical visualization. Methane was used as fuel for both the pre-chamber and the main chamber. Seven operating points were tested, including passive pre-chamber mode as a limit condition, to study the effect of pre-and main-chamber fuel addition on the pre-chamber jets and the main chamber combustion via chemiluminescence imaging. A typical cycle of one of the tested conditions is explained through the captured images. Observations of the typical cycle reveal a predominant presence of only six jets (from the lower row), with well-defined jet structures. A secondary flame front is generated in the center of the combustion chamber due to the delayed contribution of the upper row of nozzle holes. Passive mode exhibited a random and sequential jet emergence, which can be related to the observed slower burn rate. The increase in the quantity of fuel added to the main-chamber showed a strong effect on the flame's overall intensity and jet penetration. On the other hand, increasing the pre-chamber fuel injection exhibits different jet shapes.