An existing hydrocarbon emissions model has been improved to consider partial burn effects. The model previously considered only the top land piston-ring crevice and the oil film absorption/de-sorption phenomenon as hydrocarbon emissions sources, giving incorrect predictions for conditions in which incomplete combustion occurs. The model now shows the correct trends for the exhaust HC in the presence of partial burn, as has been verified in tests for very lean mixtures and for high engine speeds with retarded ignition. The experiments were carried out in a single-cylinder spark ignition engine, using isooctane as fuel. Results for variations in ignition timing, compression ratio and coolant and lubricant temperature have also been shown, following a better estimate for the wall temperature.