Engine; Idle Speed & Mixture Adjustment - AVSTAR LFR-NNSS5 Installation, Operation & Maintenance Manual

AFS-IOM-02
1. Idle Speed and Mixture Adjustment:
A. Start the engine and warm up in accordance with the Engine and Airframe
Maintenance Manuals. Prevailing winds can affect the load of the propeller and the
resulting engine RPM. Therefore, AVStar recommends setting the Idle Speed and
Mixture with the aircraft in a crosswind orientation.
B. Mag-Drop checks should be made in accordance with the applicable Engine Service
Documents, refer to Lycoming Service Instruction 1132. Once satisfactory, proceed
with idle adjustment.
C. Set the Throttle Stop Screw, sometimes referred to the Idle Speed screw, so that the
engine idles at the airframe manufacturer's recommended idling speed with a closed
throttle. Refer to Appendix A for location.
D. Once the idling speed has stabilized, move the cockpit mixture control lever with a
smooth and steady motion toward the Idle Cut-Off position and observe the
tachometer for any change during the leaning process. Caution must be exercised to
return the mixture control to Full Rich before the RPM drops to a point where the
engine cuts out. An increase of more than 50 RPM while leaning out indicates an
excessively rich idle mixture. An immediate decrease in RPM (if not preceded by a
momentary increase) indicates the idle mixture is too lean. A rise of approximately
10-50 RPM is considered satisfactory.
E. Should the Idle Mixture require adjustment after performing the previous steps, adjust
the idle mixture wheel in the direction required. The "R" engraved on the clevis
along with the arrow indicates the direction to turn the wheel to richen the mixture.
The opposite direction will lean the mixture. Refer to Appendix A for location.
F. Anytime an adjustment is made, the engine should be run up to approximately 2000
RPM to clear the engine. Upon returning to idle, it may be necessary to readjust the
Idle Speed prior to performing the next mixture rise check.
G. The procedure described above targets an idle mixture that will obtain maximum
RPM with minimum manifold pressure.
allowance and consideration should be made for the effect of weather and altitude
conditions upon idling adjustment.
H. Idle speed and mixture adjustments should not require frequent attention, aside from
cases of large variations in ambient temperature or altitude of operation. Frequent
idle adjustments may indicate or mask another issue, which may or may not be fuel
system related.
I. If the idle speed setting is not stable after repeated attempts, complete a check of the
idle linkage. Look for loose connections which could cause erratic idling.
J. AVStar DOES NOT recommend re-indexing or taking apart the idle linkage clevis
assembly. Idle fuel flows are initially set when tested on a qualified test bench, with
specific throttle positions and fuel flows. The orientation of the idle linkage is also
critical to the functionality of the internal idle valve (metering jet). If the orientation
or clocking is incorrect
AVStar Fuel Systems. All Rights Reserved
May 27, 2022
72-00 – ENGINE
Whenever this check is performed,
engine operation and performance can be affected.
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