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As the Wrench Turns


David Vital, Director of Maintenance

Adjusting Fuel Systems

Aircraft engines need a lot of adjustments to run properly.  One area that doesn’t get a lot of attention is how an engine is set to idle properly.  The numbers quoted in this article are “typical” and definitely do not apply to all airplanes, so please take this into consideration when listening to the engine idle on your next flight. These adjustments are made seasonally. Planes run richer in the summer and leaner in the winter. Most planes need minor adjustments as the season changes.  

There are several variables that can and should be set when setting up an engine idle.  First is the actual idle speed of the engine.  Typical idle speeds are in the 500-800 RPM range depending upon a whole range of individual circumstances.  Ideally the idle speed is set as low as practical such that the engine will tick over smoothly and not cough and stop. 

Next, the idle mixture can be set.  The idle mixture should be with the mixture control full forward rich. After making a setting, the mixture control should be smoothly pulled back towards lean and the engine should pick up about 50 RPM before dropping back down.  While doing the mixture pullback if the engine picks up more than 50 RPM it means that the mixture is too rich at the idle setting.  If the engine speed simply drops off when pulling the mixture back then the idle mixture setting is probably too lean.  Another test at this point is to test the throttle responsiveness.  With the mixture set to full rich, “poking” the throttle gently forward should produce an instant and smooth increase in engine speed i.e. good responsiveness to the throttle.  If there is a hesitation, it means that the idle mixture needs further adjustment.  The other thing that will happen when fiddling with the idle mixture is that it will affect the idle speed so that variable might need a subsequent adjustment.

Finally, in some cases it is appropriate and possible to adjust the fuel pressure at idle.  Typical idle fuel pressures are in the order of 7-10 psi, whereas fuel pressure at higher powers might be 20-25 psi.  This adjustment requires the temporary addition of a fuel pressure gauge to the engine while making this engine adjustment.  Setting the fuel pressure is also likely to affect the mixture and also the idle speed.  And so the adjustments go on until the idle fuel pressure is correct, the idle speed is good, the idle mixture is correct, and the throttle responsiveness is appropriate from idle to a higher power setting.