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Loss mechanisms in 1-10 kW spark-ignition, two-stroke engines may be grouped into five categories: thermal losses, frictional losses, sensible enthalpy in the exhaust gases, incomplete combustion, and short-circuiting of fresh fuel and air mixture. These loss mechanisms cause small two-stroke engines to have fuel conversion efficiencies 50%-70% lower than similar larger engines. Previous studies of loss scaling in small engines have estimated the short-circuiting using heuristics derived for larger engines or grouped it with other combustion losses to complete the energy balance. This work describes and compares two methods for measuring short-circuiting on a commercially available, two-stroke, naturally aspirated, spark ignition engine with 55 cm3 displacement. One method used oxygen as an analyte (the Watson method), nitrogen as an internal standard, and gas chromatography with a thermal conductivity detector for quantification.