IO-390-D Series Engine Maintenance Manual
© 2020 Avco Corporation. All Rights Reserved
74-30
March 2020
Page 287
IO-390-D Series Engine Maintenance Manual
K.
Install ignition harness assemblies on the magnetos. The left magneto harness is labeled
"left" and the right magneto harness is labeled "right.
L.
Torque the cap-mounting screws per the magneto manufacturer’s instructions.
M.
Install the spark plug in cylinder No. 1 per the “Spark Plug Installation” section in
Chapter 74-20.
N.
Enable power to the engine.
O.
Complete the “Operational Ground Check” in Chapter 72-00 of the engine to make sure
the magnetos are operating correctly.
3.
Magneto Adjustment Procedure
A.
Turn the crankshaft in the direction of normal rotation until the correct advance timing
mark on the starter ring gear support aligns with the reference point on the engine (Figure
8 or 9).
B.
Loosen the hold down nuts on the clamps of the magneto that is not timed with the
engine.
C.
Manually and slowly turn the magneto in its mounting flange in the direction opposite its
normal rotation until the timing light indicates the magneto is not firing.
D.
Turn the magneto in the direction of normal rotation until the timing light indicates the
magneto is firing.
E.
Torque the hold-down nuts on the magneto clamps 48 in.-lb. (5 Nm) increments,
alternating between the two nuts until both nuts are torqued to 15 to 18 ft.-lb. (20 to 24
Nm).
F.
Apply torque seal to the torqued hold-down nuts on the magneto clamps.
G.
Complete the previous steps on the other magneto to ensure that both timing lights
indicate the magnetos are firing when the timing mark on the starter ring gear support
aligns with the reference point on the engine (Figure 8 or 9).
H.
Turn the crankshaft a few degrees in the direction opposite normal rotation. The timing
lights should indicate the magnetos are not firing.
I.
Slowly turn the crankshaft in the direction of normal rotation until the correct advance
timing mark on the starter ring gear support (Figure 8) aligns with the reference point on
the engine (Figure 8 or 9). Both lights on the timing light should indicate the magnetos
are firing.
J.
Adjust the magneto-to-engine timing until repeat checks by the “Magneto-to-Engine
Timing Check” are within tolerance.
