Fuel tank caps in lawn mowers have a vent that allows air in so that fuel is easily taken out to run the engine. The vent is often fitted with a dust filter to keep dust and dirt out but allow air in. As a lawn mower vibrates, gasoline can soak the filter, which is perfectly normal.
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Michael Logan is a writer, editor and web page designer. His professional background includes electrical, computer and test engineering, real estate investment, network engineering and management, programming and remodeling company owner. Logan has been writing professionally since he was first published in Test & Measurement World in 1989.