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NOTICE: The advance timing specification in degrees is stamped on the engine data plate.
G. 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).
H. The timing light is to indicate the magneto is firing when the timing mark on the starter
ring gear support aligns with the reference point on the engine (Figures 8 or 9) to ensure
that the magnetos are correctly timed with the engine.
NOTICE: If there is interference from another engine component or an airframe
component that prevents correct adjustment of the magneto, remove the magneto
and re-position the drive gear in the accessory housing. Use care not to drop the
dampers (if installed) into the engine while repositioning the drive gear.
I. The magneto position is typically 15º to 30º above horizontal centerline (Figure 10).
NOTICE: If the magneto-to-engine timing is out of tolerance by more than 5° refer to the
magneto manufacturer's instructions for internal adjustment and then complete
the "Magneto Adjustment Procedure" in this chapter.
If the magneto-to-engine timing is out of tolerance by less than 5°, complete the
"Magneto Adjustment Procedure" in this chapter.
J. If the magneto-to engine timing is within tolerance or after the "Magneto-to-Engine
Timing Adjustment" procedure has been completed:
(1) Remove the timing light leads from the magnetos and grounding source.
(2) Attach the P-Leads to the condenser terminal of each magneto with the nut and a new
lock washer. Torque the nut to 13 to 15 in.-lb (1.5 to 1.7 Nm).
74-30
Page 286
IO-390-D Series Engine Maintenance Manual
IO-390-D Series Engine Maintenance Manual
Figure 10
Typical Magneto Position
© 2020 Avco Corporation. All Rights Reserved
March 2020
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