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.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved
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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.
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.