Rich mixture

An engine runs rich when the air-fuel mixture contains more fuel than needed for complete combustion. The opposite is a lean mixture. A slightly rich mixture is used on purpose under full load and when starting cold, because it cools the combustion chamber and helps the engine run smoothly.

Typical signs of running too rich: black, sooty spark plug, black smoke or a smell of fuel from the exhaust, higher fuel consumption, rough or 'blubbery' running and poor throttle response.

Common causes are a dirty air filter, a choke left on, the wrong jets or float level in a carburettor, a leaking injector or a faulty sensor. Check the simple things first, then the fuelling setup.