The increasing demand for sustainable and cleaner fuels has encouraged the exploration of alcohol–gasoline blends as alternatives for spark-ignition (SI) engines. This study investigates the performance and emission characteristics of a single-cylinder, four-stroke SI engine fueled with ternary blends of n-butanol, ethanol, and n-propanol mixed with gasoline. A single-cylinder, four-stroke SI engine was tested using the Varying Speed Test procedure based on PNS 396–397:2024. The tests were carried out by gradually decreasing the engine’s shaft rotation speed in 100-RPM intervals (from 2900 rpm to 3800 rpm) from its rated speed, while keeping the carburetor setting constant across all fuel blends. Five fuels were used in total, G100 (100% gasoline), BE15P0 (7.5% n-butanol, 7.5% ethanol), BE10P5 (5% n-butanol, 5% ethanol, 5% n-propanol), BE5P10 (2.5% n-butanol, 2.5% ethanol, 10% n-propanol), and BE0P15 (15% n-propanol), with each blend containing 15% alcohol by volume. This study measured engine brake power (BP), brake thermal efficiency (BTE), and brake-specific fuel consumption (BSFC), as well as emissions of CO and NO_x. Results showed that among all the blends, BE0P15 stood out by recording the highest average BTE and BP, the lowest BSFC, and the lowest emissions of both CO and NOₓ. These results suggest that fuel blends with higher amounts of n-propanol can improve combustion efficiency while also reducing harmful emissions. Based on the findings, n-propanol-rich blends have potential as cleaner and more efficient alternatives to pure gasoline in spark-ignition engines, especially under varying speed conditions.