Yamaha MM600D, Manual

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52 CHAPTER TWO
Ignited by hot deposit
PREIGNITION
Regular ignition spark
is also present. Unnoticed detonation is often the of
engine that occurs for no apparent reason.
Poor Idling
A poor idle can be caused by improper fuel mixture ad-
justment, incorrect timing or ignition system malfunc-
tions. Check the carburetor pulse and vent lines for an
obstruction. Also check for loose carburetor mounting
bolts or a faulty carburetor flange gasket.
Misfiring
Misfiring can result from a dirty spark plug. If misfiring
occurs only under heavy load, as when accelerating, it is
usually caused by a defective spark plug. Check for fuel
contamination.
Flat Spots
If the engine seems to die momentarily when the throt-
tle opens and then recovers, check for a dirty carburetor,
improperly adjusted fuel mixture or contaminated fuel.
The fuel mixture may be too rich, but usually the low
speed mixture is adjusted too lean.
Power Loss
Several factors can cause a lack of power and speed.
Problems in the fuel system to check are a clogged fuel fil-
ter, air leaks in a fuel line between the tank and the fuel
pump, a faulty fuel pump or leaking primer lines. Make
sure that
throttle slide operates properly.
The ignition timing is fixed and not adjustable. How-
ever, a faulty ignition system component may cause
Flame fronts collide
and result in very
rapid expansion
TDC
Total expansion
occurs near top
of stroke
rect ignition timing, which can result in a power loss.
Preignition or detonation also causes a power loss.
A galling piston or cylinder, incorrect piston clearance
or worn or sticky piston rings may be responsible. Look
for loose bolts, defective gaskets or leaking mating
faces on the cylinder head, cylinder or crankcase. Also
check the crankshaft seals. Refer to Two-Stroke Pressure
Testing in this chapter.
Exhaust fumes leaking within the engine compartment
can slow and stop the engine.
Refer to Figure 23 for a general listing of engine trou-
bles.
Piston Seizure
Piston seizure or galling is the transfer of metal from the
piston to the cylinder bore. Friction and the resultant heat
causes piston seizure. Some causes may be pistons with
incorrect bore clearances, piston rings with an improper
end gap or a compression leak. Incorrect type of oil, lack
of oil or an incorrectly operating oil injection pump may
cause this problem. Incorrect spark plug (wrong heat
range), incorrect ignition timing or overheating from any
cause may result in piston seizure.
A reduction of speed may be the first sign of
seizure while immediate stoppage indicates full lockup. A
rattling noise from the engine top end is often an early
sign of seizure.
When diagnosing piston seizure, the pistons themselves
can be used to troubleshoot and determine the cause. High
cylinder temperatures normally cause seizure above the
piston rings while seizure below the piston rings is usually
caused by a lack of proper lubrication.
See Figure
24 and Figure 25 for examples of piston
seizure.