Yamaha MM700A Manual page 84

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3. Over-retarded ignition timing.
4. Ignition component failure.
5. Low engine compression.
fouled
The tip of an oil fouled plug has a black insulator tip, a
damp oily film over the firing end and a carbon layer over
the entire nose. The electrodes will not be worn. Common
causes for this condition are:
1. Excessive oil in the fuel (incorrect jetting or incorrect
oil pump adjustment).
2. Wrong type of oil.
4. Cold spark plug heat range.
5. Engine still being broken in.
Oil fouled spark plugs may be cleaned in an emergency,
but it is better to replace them. It is important to correct the
cause of fouling before the engine is returned to service.
Gap bridging
Plugs with this condition exhibit gaps shorted out by
combustion deposits between the electrodes. If this condi-
tion is encountered, check for an improper oil type, exces-
sive carbon in the combustion chamber or a clogged
exhaust port and pipe. Make sure to locate and correct the
cause of this condition.
Overheating
Badly worn electrodes and premature gap wear are
signs of overheating, along with a gray or white blistered
porcelain insulator surface. The most common cause for
this condition is using a spark plug of the wrong heat
range (too hot). If the plug has not been changed to a hot-
ter spark plug and the plug is overheated, consider the fol-
lowing causes:
1. Lean fuel mixture (incorrect main jet or incorrect oil
pump adjustment).
3. Cooling system malfunction.
4. Engine air leak.
5. Improper spark plug installation (overtightening).
Worn out
Corrosive gases formed by combustion and high volt-
age sparks have eroded the electrodes. Spark plugs in this
condition require more voltage to fire under hard acceler-
ation. Replace with a new spark plug.
Preignition
If the electrodes are melted, preignition is almost cer-
tainly the cause. Check for carburetor mounting or intake
manifold leaks and overadvanced ignition timing. It is
also possible that a plug of the wrong heat range (too hot)
is being used. Find the cause of the preignition before re-
turning the engine into service.
Ignition Timing
All models are equipped with a capacitor discharge ig-
nition (CDI). This system uses no breaker points and
CHAPTER THREE