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:

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 160-190 degree range.

Second Step:

1. Mixture to full rich.
2. Set engine RPM to 700. The carb needs to be out of its IDLE mixture circuit. The engine should IDLE smoothly.
3. Lean the mixture to peak RPM.  Be patient.  We need to find where the mixture is leanest.
4. Close the throttle. (i.e., throttle to IDLE)
5. Using the throttle stop screw, set the throttle speed to IDLE RPM plus the desired RPM rise. (e.g., 575 rpm for a TIger at Sea Level)

Desired RPM rise: (this is a reference RPM value.  it just needs to be close.)

Sea Level:  75 RPM
1000 MSL:  60 RPM
2000 MSL:  50 RPM
3000 MSL:  40 RPM
4000 MSL:  30 RPM
5000 MSL:  20 RPM
6000 MSL:  10 RPM (just about any rise is sufficient)

Third Step:

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, bummer. 

Is the IDLE RPM too HIGH?  (e.g., higher than 500 RPM)  go to Fourth Step.
Is the IDLE RPM too LOW?  (e.g., Lower than 500 RPM)  go to Fifth Step.

Fourth Step:
(IDLE too HIGH)

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 (finding the RPM rise)
6. Go to Third Step (mixture to full rich. finding the IDLE RPM)

Fifth Step:
(IDLE too LOW)

1. This is an indication that the IDLE mixture screw is too rich.
2. Turn the IDLE mixture screw in 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 (finding the RPM rise)
6. Go to Third Step (mixture to full rich. finding the IDLE RPM)

Sixth Step:

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

1. For really tough cases, go fly the plane and run it lean to clean the plugs.  A Tiger should IDLE smoothly at 450 RPM.
2. When you get back, before the plugs get dirty, with the mixture at full rich . . . .
3. Throttle to IDLE.  Note RPM.
4. Slowly pull the mixture lever out until the RPM rises.

Is the IDLE RPM too high?  (e.g., higher than 500 RPM)  go to Fourth Step.
Is the IDLE RPM too Low?  (e.g., Lower than 500 RPM)  go to Fifth Step.

5. Repeat as necessary.  Good Luck.  I would estimate that 90% of the planes I get are set up wrong.