Tuning Tips
Adjustments for Competition 155
Tuning Tips
Refer to Spark Plug on page 78.
The following procedure is recommended.
You may not get an accurate reading if you simply
turn off the engine and pull the plug for inspection.
Use a new spark plug. Inspect the plug before
installing it.
Using spark plugs with an improper heat range or
incorrect reach can cause engine damage.
Ride for 10 – 15 minutes before taking a plug
reading. A new plug will not color immediately.
Before removing the spark plug, clean the spark
plug area thoroughly to prevent dirt from entering
the cylinder.
To obtain an accurate reading of a new spark plug:
1. Accelerate at full throttle on a straight.
2. Depress and hold the engine stop button and
pull the clutch lever in.
3. Coast to a stop.
4. Remove the spark plug.
5. Use a magnifying glass to inspect the spark
plug. The porcelain insulator (1) around the
center electrode (2) should appear clean and
colorless with a gray ring around the center
electrode where it exits the porcelain.
Light gray or white color streaks the porcelain
insulator and center electrode indicate lean air-
fuel mixture. Wet or black sooty streaks on the
porcelain indicate rich air-fuel mixture.
An improperly tightened spark plug can damage
the engine. If a plug is too loose, the piston may be
damaged. If a plug is too tight, the threads may be
damaged.
Spark Plug Coloring Guidelines
Remember that in addition to improper air-fuel
mixture:
• A lean condition can be caused by air leaks in
the inlet tract or exhaust system, the passage of
too much air because of the use of the wrong
air cleaner, or use of a less-restrictive
aftermarket exhaust system.
• A rich condition can be caused by a plugged or
dirty air cleaner, use of a more-restrictive
aftermarket exhaust system, or excessive oil on
the air cleaner.
Excessive smoking will occur.
Spark Plug Reading
(1) porcelain insulator (2) center electrode
Condition Spark Plug Appearance Mixture
Normal Dark brown to light tan
color with dry electrode
correct
Overheating
(Lean)
Light gray or white color lean
Wet (Rich) Wet or sooty rich
(2)
(1)
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