Setting the Engine IDLE

Setting the Engine IDLE and IDLE mixture is not as straight forward as one might think.  The problem comes from the interaction between intake manifold vacuum, AKA manifold absolute pressure, and the location of the IDLE fuel slot in the carburetor venturi.  A small change in the IDLE set screw changes the signal to the IDLE mixture circuit.  Changing the IDLE mixture changes IDLE RPM, which changes the signal to the IDLE circuit.  See what I mean?

First Step: (Warm up the engine)

1. Start the engine, go fly if possible.
2. Verify the oil temperature of at least 100 degrees F.  The ideal would be a temp in the 160 degree range.

Second Step: (Engine RPM)

1. Mixture to full rich.

2. Set engine RPM to 700. The engine should IDLE smoothly.
3. Lean the mixture to peak RPM.

4. Using the throttle stop screw, reduce the throttle to IDLE RPM plus the desired RPM rise.
. . . Note: Desired RPM rise
. . . . . . . .Sea Level: 75 RPM

. . . . . . . .1000 MSL: 60 to 65 RPM

. . . . . . . .2000 MSL: 45 to 50 RPM

. . . . . . . .3000 MSL: 30 to 35 RPM

. . . . . . . .4000 MSL: 20 to 25 RPM

5. Verify the mixture is at peak RPM.

Third Step: (IDLE RPM at full rich)

1. Move the mixture to full rich.
2. IDLE RPM should go to desired value.
3. Piece-o'-cake. You're done. Go to Sixth Step.
4. If not, go to Fourth Step

Fourth Step: (If the IDLE RPM is too high.  Otherwise, if it's too low, skip to Fifth Step.)

1. This is an indication that the IDLE mixture screw is too lean.
2. Back out the IDLE mixture screw by 1/2 turn.
3. Observe the IDLE RPM.
4. Repeat 2 until the RPM is what you want.   Subsitute smaller mixture screw changes if necessary.
5. Go to Second Step
6. Go to Third Step

Fifth Step: (If the IDLE RPM is too low)

1. This is an indication that the IDLE mixture screw is too rich.
2. Screw in the IDLE mixture screw by 1/2 turn.
3. Observe the IDLE RPM.
4. Repeat 2 until the RPM is what you want.   Subsitute smaller mixture screw changes if necessary.
5. Go to Second Step
6. Go to Third Step

Sixth Step: (I'll bet you thought you were done.)

1. Odds are, you're very close now.
2. Run the throttle up 1500 RPM or so for about a minute.
3. Close the throttle.
4. Observe the IDLE RPM.  Make a note of the RPM.
5. Slowly back out the mixture toward IDLE CUTOFF.

6. Observe the IDLE RPM rise.  Make a note of the rise.
7. If you get the IDLE RPM and the RPM rise you wanted. You're done. If not, go to Seventh Step

Seventh Step: (Repeat)

1. Restart the engine
2. For really tough cases, go fly the plane and run it lean to clean the plugs.
2. Otherwise, run the engine up to 1500 RPM for a minute or so.
4. Throttle to IDLE
5. Repeat Second Step
6. Repeat Third Step