A fast response electromechanical sampling valve has been used to collect samples from the exhaust of a spark ignition engine. The variations in concentration of unburned fuel and other hydrocarbon species through the engine cycle have been analyzed by gas chromatography. The results have pointed out that most of the unburned fuel comes from sources located in the lower part of the combustion chamber, such as piston-ring crevices and oil film absorption/desorption. The appearance of non-fuel HC species in the exhaust have been related to the post-flame oxidation process of the unburned fuel. A high-frequency flame ionization detector (fast FID) was employed to measure the cyclic variation of the total exhaust hydrocarbons.