Carbon monoxide concentrations in the exhaust from an engine operated under Computer Command Control were measured with infrared diode laser spectroscopy. CO levels were measured before and after the catalytic converter for various air-fuel ratios, using two different carburetors and using both analog and digital carburetor controllers. The response time of the spectrometer was 0.025 s, which was fast enough to resolve the rapidly varying CO concentrations. Using signal-averaging techniques the CO component was isolated which was due to the modulation of the carburetor's fuel metering rod. A component at the firing frequency of a single cylinder was also isolated; this component results from maldistribution of fuel to the engine's eight cylinders. Fourier transforms of the data yielded relative amplitudes of CO oscillation as a function of frequency.