Mercury 305 CID, Service Manual, Page: 78 / 967

1C-18 - TROUBLESHOOTING 90-823225--1 1096
Engine Noise
Important Information
No definite rule or test will positively determine
source of engine noise; therefore, use the following
information only as a general guide to engine noise
diagnosis.
1. Use a timing light to determine if noise is timed
with engine speed or one-half engine speed.
Noises timed with engine speed are related to
crankshaft, rods, pistons, piston pins, and fly-
wheel. Noises timed to one-half engine speed
are valve train related.
2. The use of a stethoscope can aid in locating a
noise source; however, because noise will travel
to other metal parts not involved in the problem,
caution must be exercised.
3. If you believe noise is confined to one particular
cylinder, ground spark plug leads, one at a time.
If noise lessens noticeably or disappears, it is iso-
lated to that particular cylinder.
4. Try to isolate the noise to location in engine: front
to back, top to bottom. This can help determine
which components are at fault.
5. Sometimes noises can be caused by moving
parts coming in contact with other components.
Examples are: flywheel or coupler; exhaust
flappers rattling against exhaust pipe; crankshaft
striking (pan, pan baffle, or dipstick tube); rocker
arm striking valve cover; and loose flywheel
cover. In many cases if this is found to be the
problem, a complete engine teardown is not
necessary.
6. When noise is isolated to a certain area and com-
ponent, removal and inspection will be required.
Refer to proper sections of service manual for in-
formation required for service.
7. If noise cannot be distinguished between engine
and drive unit, remove drive from boat. Run a
water supply directly to engine. Run engine
without the drive to determine if noise is still there.
Valve Cover Area
Location Possible Cause
V l i d h lf i
Rocker arm striking valve cover
Valve cover area timed to one-half engine
Rocker arm out of adjustment
Valve cover area, timed to one-half engine
speed, noise could be confined to one cylinder
or may be found in any multitude of cylinders
Worn rocker arm
or may be found in any multitude of cylinders
Bent push rod
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