<P> Ways to check carburetor mixture adjustment include: measuring the carbon monoxide, hydrocarbon, and oxygen content of the exhaust using a gas analyzer, or directly viewing the color of the flame in the combustion chamber through a special glass - bodied spark plug sold under the name "Colortune"; the flame color of stoichiometric burning is described as a "Bunsen blue", turning to yellow if the mixture is rich and whitish - blue if too lean . Another method, widely used in aviation, is to measure the exhaust gas temperature, which is close to maximum for an optimally adjusted mixture and drops off steeply when the mixture is either too rich or too lean . </P> <P> The mixture can also be judged by removing and scrutinizing the spark plugs . Black, dry, sooty plugs indicate a mixture too rich; white or light gray plugs indicate a lean mixture . A proper mixture is indicated by brownish - gray / straw - coloured plugs . </P> <P> On high - performance two - stroke engines, the fuel mixture can also be judged by observing piston wash . Piston wash is the color and amount of carbon buildup on the top (dome) of the piston . Lean engines will have a piston dome covered in black carbon, and rich engines will have a clean piston dome that appears new and free of carbon buildup . This is often the opposite of intuition . Commonly, an ideal mixture will be somewhere in - between the two, with clean dome areas near the transfer ports but some carbon in the center of the dome . </P> <P> When tuning two - strokes It is important to operate the engine at the rpm and throttle input that it will most often be operated at . This will typically be wide - open or close to wide - open throttle . Lower RPM and idle can operate rich / lean and sway readings, due to the design of carburetors to operate well at high air - speed through the Venturi and sacrifice low air - speed performance . </P>

What is the purpose of the accelerator pump fitted to the carburettor of a piston engine