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4. Move the throttle stick up to full (open) throttle. If the engine runs rough or if some cylinders
are not operating, the fuel mixture is too rich.
5. Lower the throttle stick until the engine reaches 2,000 rpm. Lean the high-speed needle
(turn clockwise) 1/16 turn.
6. Quickly raise the throttle stick to full (open) throttle. If the engine stops running,
the high-speed needle setting is too lean.
7. Lower the throttle stick until the engine reaches 2,000 rpm. Richen the high-speed
needle (turn counterclockwise) 1/8 turn.
8. Repeat Steps 4–7 until the engine is running reliably at full (open) throttle.
High-Speed Needle Tuning
Once the engine is running reliably at full (open) throttle:
1. Lean the high-speed needle 1/16 of a turn at a time.
2. Wait 2–3 seconds for the engine to respond. Radial engines do not respond immediately
to tuning changes.
a. If there is an increase in RPM: Lean the high-speed needle an additional 1/16 turn and
wait for the engine to respond to the tuning change. If there is no increase in RPM
after the change, turn the high-speed needle back to its previous position.
b. If there is a decrease in RPM: Richen the high-speed needle 1/8 turn and wait for
the engine RPM to stabilize.
3. Repeat Step 2 until you determine the maximum RPM with the fuel and propeller
you selected.
4. Richen the high-speed needle so when full throttle is applied, the engine rpm will be steady
(not drop off) after reaching its peak RPM. Richening the high-speed needle allows the RPM
to increase when the airplane is flying.
Low-Speed Needle Adjustment
Low-speed needle adjustment controls the fuel/air mixture at idle and the quality
of the transition between idle and wide open throttle.
Low-Speed Needle Tuning
1. Adjust the high-speed needle for the current conditions.
2. Allow the engine to idle for 10–15 seconds. Rapidly advance the throttle from idle
to full open.
If the engine transitions with no hesitation, the low-speed needle is adjusted perfectly.
If the engine slowly responds to throttle change and gradually reaches peak RPM, the
low-speed needle is too rich. Lean out the low-speed needle (turn clockwise) 1/16
turn and test again.
If the engine immediately stops running, the low-speed needle is lean. Richen the low-speed
needle 1/16 turn and test again.
3. When the low-speed needle is adjusted correctly, check the high-speed needle setting again.
4. When you are satisfied with the needle settings, you should not need to adjust the low-
speed needle again. The low-speed needle setting is mechanically controlled and does
not change with atmospheric conditions.
The best indication that your engine is tuned properly is the color of your spark plugs when
removed from the cylinder. A nice dry tan color is an indicator that the fuel mixture is correct
and burning well. Anything else (wet plugs, or carbon deposits) generally indicates you are
running the engine too rich.
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