Setting the Engine Idle

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Setting the Engine Idle (This was transcribed from Light Plane Maintenance)

1. Warm up the engine
2. Set the IDLE (in the cockpit) to 700 RPM with the mixture full rich
. . . . . . . . . It should idle smoothly. If not, adjust the cockpit mixture to smooth out the Idle.
3. Lean the cockpit mixture to peak RPM (lowest manifold pressure) Keep RPM below 1000 RPM.
4. Reduce throttle to the IDLE RPM (specified in the airframe manual) plus desired rpm rise.

For a 75 RPM rise at sea-level, set the IDLE to
675 - 725 RPM for an AA5A with a specified IDLE of 600-650 RPM
. . . . . or
575 - 725 RPM for an AA5B with a specified IDLE of 500-650 RPM

NOTE: Decrease specified RPM 10 - 15 RPM per 1000 feet of Altitude.

5. Recheck mixture to verify setting is at peak RPM.
6. Set the IDLE speed screw to this Idle position.
7. Manually richen the cockpit mixture control to the full rich position.
. . . - the IDLE speed should drop to the desired RPM.
. . . - if the IDLE speed stays higher than desired, richen the Idle mixture on the Carb
. . . . until the desired engine RPM is achieved.

NOTE: Some MA-4 series carb will only have a 25 RPM rise at altitude.

Example: At Fox Field, Elevation 2300 feet. (25-35 RPM lower)

Set IDLE in step 4 to

650 - 700 RPM for an AA5A with a specified IDLE of 600-650 RPM
. . . . . or
550 - 700 RPM for an AA5B with a specified IDLE of 500-650 RPM

NOTE: Remember to decrease specified RPM 10 - 15 RPM per 1000 feet of Altitude.

Set Carb mixture in step 7 for an IDLE speed of

575 - 625 RPM for an AA5A with a specified IDLE of 600-650 RPM
. . . . . or
475 - 625 RPM for an AA5B with a specified IDLE of 500-650 RPM

NOTE: I've tried this technique with my Cheetah. The jamb nut on the cable (at the throttle end) won't let me set the Idle below about 650 RPM when it should be idling around 625 RPM. (I have a digital tach and I can monitor the RPM changes very closely.) When leaning to peak RPM, I can expect about a 25 RPM rise. Likewise, when I shut down using the mixture control, I usually get about a 25-35 RPM rise. I am going to try and reset the Idle again next annual.