Spark Plug Fouling - Lycoming IO-390-D Series Maintenance Manual

IO-390-D Series Engine Maintenance Manual
IO-390-D Series Engine Maintenance Manual
C. To be acceptable, the spark plug must not have any of the following defects:
(1) Fine wire plugs with loose center or ground electrodes
(2) Electrodes that show signs of metal or impact damage
(3) Massive electrode plugs with copper run-out of the center electrode
(4) Ceramic core nose with a cracked or crazed rough surface
D. Measure the spark plug gap to make sure it is at the correct tolerance per the latest revision of
Service Instruction No. SI-1042. Reset the spark plug gap if it is not correct. Refer to the
"Spark Plug Gap Setting" procedure in this chapter.
Spark Plug Findings
Acceptable Insulator tip gray, tan
Spark Plugs or light brown
No ash deposits
Electrodes intact, not
burnt or eroded
Partially Ash deposits
Worn Spark Electrode burnt
Plugs and/or eroded to less
than half of the
original thickness
More voltage has
been necessary to fire
the spark plugs
Worn Spark Erosion of center and
Plugs ground electrode
Extensive necking of
the fine wire ground
electrode

5. Spark Plug Fouling

Lead deposits can collect on the spark plug electrodes when the engine operates at lower-
A.
than-specified temperatures with fuel-rich mixtures (fuel-rich mixtures do not enable
vaporization of lead in aviation gas). These deposits can cause misfiring.
© 2020 Avco Corporation. All Rights Reserved
March 2020
General Spark Plug Wear/Replacement Guidelines
Condition of Fine
Wire Ground
Electrode on Spark
Plug
Table 1
Condition of Center
Electrode on Spark
plug
What to do
Clean, set the
spark plug gap and
install the spark
plug per applicable
sections in this
chapter and in
Chapter 05-30.
Discard the spark
plug and replace
with a serviceable
spark plug.
Look for excessive
heat sources.
Discard the spark
plug and replace
with a serviceable
spark plug.
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