Mariner 30 JET 40 (4 CYL), Service Manual

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90-814676R1 DECEMBER 1996 POWERHEAD - 4-29
d. Chatter Marks: Chatter marks are the result
of a combination of low speed-low load-cold
water temperature operation that is aggra-
vated by inadequate lubrication and/or im-
proper fuel. Under these conditions, the
crankshaft journal is hammered by the con-
necting rod. As ignition occurs in the cylinder,
the piston pushes the connecting rod with tre-
mendous force, and this force is transferred
to the connecting rod journal. Since there is
little or no load on the crankshaft, it bounces
away from the connecting rod. The crank-
shaft then remains immobile for a split se-
cond, until the piston travel causes the con-
necting rod to catch up to the waiting
crankshaft journal, then hammers it. The rep-
etition of this action causes a rough bearing
surface(s) that resembles a tiny washboard.
In some instances, the connecting rod crank
pin bore becomes highly polished. During op-
eration, the engine will emit a “whirr” and/or
“chirp” sound when it is accelerated rapidly
from idle speed to approximately 1500 RPM,
then quickly returned to idle. If the preceding
conditions are found, replace both the crank-
shaft and connecting rod.
51853
a
a - Chatter Marks between Arrows
e. Uneven Wear: Uneven wear could be
caused by a bent connecting rod or improper
shimming of crankshaft end play [failure to
maintain approximately the same amount of
shim(s) under end cap, thereby causing the
crankshaft journal not to be centered over the
cylinder bore].
51853
a
a - Uneven Wear between Arrows
f. Overheating: Overheating is visible as a blu-
ish bearing surface color caused by inade-
quate lubrication or excessive RPM.