REFLECTOR: Idle Adjustment for Fuel Injected Lycomings

[Ron Brown]

I have retyped the text from Mack's and Andy's document (it was a bit hard to read!)

IDLE SPEED AND MIXTURE ADJUSTMENT

5-2. Check magnetos in accordance with instructions furnished in the aircraft operational manual. If the "mag-drop" is normal, proceed with idle adjustment.

5-3. Close the throttle to idle (approximately 650 rpm). If the rpm changes appreciable after making the idle mixture adjustment during the succeeding steps, readjust the idle speed to the desired RPM.

5-4. When the idle speed has been stabilized, move the cockpit mixture control lever with a smooth, steady pull, into the IDLE CUT-OFF position an observe the tachometer for  any change during the leaning out process. Caution bust be exercised to return the mixture control to the FULL-RICH position before the rpm can drop to a point where the engine cuts out. An increase in rpm while leaning out indicates the idle mixture is on the rich side of best power. An immediate decrease in rpm (if not preceded by a momentary increase) indicates that the idle mixture is on the lean side of best power.

5-5. The desired idle setting is rich enough to provide a satisfactory acceleration under all conditions and lean enough to prevent spark plug fouling or rough operation. A rise of 25-50 rpm will usually satisfy both of these conditions.

5-6. If step 5-3 indicates that the idle adjustment is too rich or too lean, turn the idle mixture adjustment one or two notches in the direction required for correction and check this new position by repeating step  5-3. Make additional adjustments as necessary.

5-7. Each time an adjustment is changed, clear the engine by running it up to approximately 2000 rpm before making mixture check.

5-8. The actual idle mixture adjustment is made by the lengthening (richening) or shortening (leaning) of the linkage between the throttle lever and idle valve lever. The center screw assembly has right hand threads on both ends but one end has a No. 10-24 thread and the other end has a No. 10-32 thread. For easy reference, consider only the coarse threaded end. When it is turned out of hits block, the linkage becomes longer and a richer mixture is provided. When it is turned into its block, the linkage is shortened and a leaner mixture is provided.

5-9. A major adjustment is available for use when the center screw bottoms out on either of the blocks. If the idle adjustment is almost satisfactory, measure the distance between the two blocks. Disconnect the spring from the most accessible linkage pin and remove the pin. Turn the block and adjustment screw until the adjusting wheel is centered and the distance between the blocks is as previously measured. There is now additional adjustment range and the reference point is retained.

5-10. Make the final idle speed adjustment to obtain the desired idling rpm with closed throttle.

5-11. If the setting does not remain stable, check the idle linkage; any looseness in this linkage will cause erratic idling. In all cases, allowance should be made for the effect of weather conditions upon idling adjustment. The relationship of the aircraft to the direction of the prevailing wind will have an effect on the propeller load and its rpm; hence it is advisable to make the idle setting with the aircraft cross-wind.

5-12. Idle speed and mixture adjustments made according to this method should require little further attention except for extreme variations in temperature and altitude.
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