The operation of a small size cogeneration gas engine (6 cylinders, 122 mm bore, 142 mm stroke) modified for unscavenged prechamber ignition was experimentally investigated. The objective was to evaluate the potential to reduce the exhaust gas emissions, particularly the CO emissions, below the Swiss limits (NOX and CO emissions: 250 and 650 mg·m−3N, 5% O2, respectively), without exhaust gas after treatment, through variations of the prechamber geometrical configuration (size, number, distribution and orientation of the nozzle orifices and prechamber internal volume and shape). The results indicate that trends which increase the penetration of the gas jets and/or promote an early arrival of the flame front at the piston top land crevice entrance (small total nozzle orifice cross sectional area, limited number of nozzle orifices, an orientation of the nozzle orifices towards the squish region, relatively large prechamber internal volume) are beneficial to reduce the CO and THC emissions. Further, these trends mainly result in a slight increase in fuel conversion efficiency.