Statistically designed experiments were run in a single-cylinder engine to understand the reason for the decrease in exhaust mass HC emissions found in the Auto/Oil Program with decreasing 90% distillation temperature (T90) of gasoline. Besides T90, the effects of mixture preparation, equivalence ratio, and ambient temperature on emissions and fuel consumption were measured. HC emissions were higher with PFI than with premixed charge, but decreasing T90 decreased HC emissions with both premixed charge and PFI. Rich mixture and low ambient temperature increased HC emissions. Speciated exhaust HC measurements indicate that incomplete vaporization of heavy components of the gasoline (C8-C10 alkanes, C6-C9 aromatics and alkenes) was responsible for higher HC emissions.