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This paper describes the development of diagnostics and testing methods for the characterization of carburetor exit flow conditions in small utility engines. These diagnostics include: 1) Three different methods of acquiring intake flow photographs. 2) A technique to measure the thickness of the fuel film running along the bottom of the intake manifold using the electrical properties of the fuel. 3) A system for measuring the A/F ratio across the carburetor exit using a heated catalyst to oxidize the sampled mixture and a wide-range oxygen sensor to determine the A/F of the reacted sample.

Three different carburetor types have been tested to observe differences in the characteristics of the fuel/air mixtures produced. To characterize the fuel/air mixtures, two diagnostics have been applied: 1) High speed movies and subsequent analysis of the exit flow, and 2) measurement of the A/F ratio found in different positions within the intake manifold. The three different carburetor types that have been studied include a fixed-venturi, fixed-jet butterfly carburetor, a slide-valve carburetor, and a constant-velocity carburetor. Each carburetor type produced a unique set of exit flow characteristics, with differences in the optical density of fuel exiting the carburetor, and differences in the apparent amount of fuel on the intake manifold wall, entrained in the air flow, and in vapor phase.