A secondary air pump for a small internal combustion engine for supplying secondary air to the exhaust system of the engine upstream of a catalyst element. The air pump includes a thinly-profiled pump housing attached to the crankcase of the engine, an inlet in fluid communication with the atmosphere, and an outlet in fluid communication with the exhaust system of the engine upstream of the catalyst element. First and second impellers within the pump housing and are driven by the crankshaft and camshaft of the engine, respectively, and cooperate with one another and with the pump housing to define a plurality of moving working spaces which convey air from the inlet to the outlet during running of the engine.

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