This paper presents a study on the energy efficiency and emissions of a converted high-revolution bore 79.5 mm/stroke 95 mm engine with a conventional fuel injection system for operation with dual fuel feed: diesel (D) and natural gas (NG). The part of NG energy increase in the dual fuel is related to a significant deterioration in energy efficiency $\left({\eta }_i\right)$ , particularly when engine operation is in low load modes and was determined to be below 40% of maximum continuous rating. The effectiveness of the D injection timing optimisation was established in high engine load modes within the range of a co-combustion ratio of NG $\le$ 0.4: with an increase in ${\eta }_i$ , compared to D, the emissions of ${\mathrm{NO}}_{\mathrm{x}}+$ HC decreased by 15% $\mathrm{to}$ 25%, while those of ${\mathrm{CO}}_2$ decreased by 8% to16%; the six-fold $\mathrm{CO}$ emission increase, up to 6 g/kWh, was unregulated. By referencing the indicated process characteristics of the established NG phase elongation in the expansion stroke, the combustion time increase as well as the associated decrease in the cylinder excess air ratio (α) are possible reasons for the increase in the incomplete combustion product emission.