Ammonia is one of the carbon-free alternatives considered for power generation and transportation sectors. But ammonia’s lower flame speed, higher ignition energy, and higher nitrogen oxides emissions are challenges in practical applications such as internal combustion engines. As a result, modifications in engine design and control and the use of a secondary fuel to initiate combustion such as natural gas are considered for ammonia-fueled engines. The higher-octane number of methane (the main component in natural gas) and ammonia allows for higher compression ratios, which in turn would increase the engine's thermal efficiency. One simple approach to initiate and control combustion for a high-octane fuel at higher compression ratios is to use a spark plug. This study experimentally investigated the operation of a heavy-duty compression ignition engine converted to spark ignition and ammonia-methane blends. Engine operation at a 40% natural gas energy substitution rate with ammonia was compared with methane-only engine operation at low speed (900 rpm), lean conditions (equivalence ratio of 0.85) and medium load (7 bar indicated mean effective pressure). The spark timing was varied from -25 CA to -10 CA ATDC to evaluate if a change in spark timing is needed when ammonia is added. Results show that, as expected, advancing the spark timing led to increased and advanced peak pressure and heat release rate, and increased nitrogen oxides emissions. Then, replacing the natural gas with ammonia decreased and retarded in-cylinder pressure due to the associated decrease in the flame propagation rate. However, replacing the natural gas with ammonia at a 40% energy replacement level did not affect the indicated mean effective pressure, thermal efficiency, or the optimum spark timing of -20 CA ATDC, but NOx emissions increased substantially.