idle mixture

idle mixture

The fuel-air mixture used when a reciprocating engine is at idling speed. This mixture is richer than that required for the best burning because of an uneven distribution of the fuel-air mixture at idle speeds.
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But if you're stuck in the carb age, tweaking the idle mixture (perhaps often) for the intended operating climate could be a necessary evil for properly maintaining it.
Lycoming even suggests resetting idle mixture on a seasonal basis, if you live in an area with wide swings in density altitude.
As for symptoms that suggest it's time for an idle mixture adjustment, you might observe the engine running rich in summer and lean in winter temperatures.
Conversely, if you saw little or no RPM rise, your idle mixture is set too lean.
Of course, alterations in idle mixture have an effect on idle speed as well.
Obviously, several iterations of the basic procedure may well be necessary to get the carb set up correctly for both idle mixture and idle speed, since one affects the other.
Turned out the carburetor was missing its idle mixture adjustment screw, causing a huge induction leak in the idle range.
If a truck is equipped with the wrong carburetor, proper tuning may be literally impossible, yet the correct adjustment of ignition timing, idle speed, idle mixture, and power mixture are critical.
Examples of measures taken then to control these gases are: lowered compression ratio, which generally decreases crevice HC and raises post-combustion gas temperature to promote oxidation of HC and CO; retarded spark to raise post-combustion gas temperature; increased idle speed, accompanied by a leaner idle mixture, for lower HC and CO; slowed throttle closing on decelerations; preheated intake air to a thermostatically controlled temperature for maintenance of a more uniform air-fuel ratio schedule under various ambient conditions; and added air to the exhaust from a belt-driven pump to promote oxidation of HC and CO.