Engine - Yamaha MM700A Manual

TROUBLESHOOTING
4. Flat spots are felt during operation that feel much like
the engine is trying to run out of gas.
5. Engine power is reduced.
6. At full throttle, engine rpm does not hold steady.

ENGINE

Engine problems are generally symptoms of something
wrong in another system, such as ignition, fuel or starting.
If properly maintained and serviced, the engine should ex-
perience no problems other than those caused by age and
wear.
Overheating and Lack
of Lubrication
Overheating and lack of lubrication causes major en-
gine mechanical damage. Make sure the cooling system is
not damaged and the oil injection tank is always filled.
Make sure the cooling fluid has antifreeze that is properly
mixed and in the proper ratio to protect at the tempera-
tures encountered. Make sure the thermostat is opening
correctly. Incorrect ignition timing, a faulty cooling sys-
tem or an excessively lean fuel mixture can also cause the
engine to overheat. The wrong spark plug may cause
preignition and overheating.
Jumper cable
Jumper cable
Starter
motor
Preignition
Preignition is the premature burning of fuel and is
caused by hot spots in the combustion chamber (Figure
21). The fuel actually ignites before it should. Glowing
deposits in the combustion chamber, inadequate cooling
or overheated spark plugs can all cause preignition. This
is first noticed in the form of a power loss but eventually
results in extended damage to the internal parts of the en-
gine because of higher combustion chamber tempera-
tures.
Detonation
Commonly called spark knock or fuel knock, detona-
tion is the violent explosion of fuel in the combustion
chamber instead of a controlled burning and expansion
(Figure 22). The excessive combustion pressure can
cause severe mechanical damage. Using low octane gaso-
line is a common cause of detonation, but detonation can
still occur when using high octane gasoline.
Some other causes of detonation are improper ignition
timing, lean fuel mixture, inadequate engine cooling,
cross firing of spark plugs, or the excessive accumulation
of deposits on the piston and in the combustion chamber.
If the engine has been modified, the compression ratio
may be too high or a piston with an improperly designed
squish area may have been installed.
Detonation may not be noticed, especially on liq-
uid-cooled engines, at high engine rpm when wind noise
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