MTD P90 series Shop Manual page 25

V. Makes unusual smoke when running
a. Black smoke, usually heavy, usually indicates a rich air fuel mixture
• Not enough air: air flow blockage or a partially closed choke.
• Too much fuel: carburetor float or float valve stuck or metering / emulsion issues with the car-
buretor.
b. White smoke, usually heavy
• Oil in muffler, usually the result of improper tipping. The engine will "fog" for a minute or so,
then clear-up on its own.
• Massive oil dilution with gasoline. It may be caused by improper tipping. It can also be caused
by leaky carburetor float valve, if there is a down-hill path from the carburetor to the intake port.
Check oil for gasoline smell, repair carburetor.
c. Blue smoke, usually light.
PCV system
• May be blocked or unplugged.
• May be over-come by massive over-filling or oil dilution with gasoline.
• Will cause oil to exit the engine via any low-resistance paths.
Piston rings
• Confirm with leak-down test.
• Smoke will be more pronounced under load.
• Repair may not make economic sense.
Valve guides (and intake valve stem seal).
• Smoke will be more pronounced on over-run.
VI. Makes unusual noise when running
a. Knock
• Check for loose mounting of engine or driven implement
• Rotate crankshaft back-and-forth to check for loose connecting rod.
b. Click
• Clicks and pops on engine shut-down: Compression release coming into play as the engine
RPMs cross the activation threshold. This will have no ill effects on engine performance.
• Half-engine speed clatter: loose valve lash.
• Half-engine speed clatter, slightly heavier: wrist-pin.
• Rhythmic heavy-light engine speed click: piston slap
c. Spark-knock
• Advanced ignition timing
• Low octane fuel
• Over-heating engine (check for blocked cooling air flow)
• Carbon build-up in cylinder: glowing carbon chunks pre-igniting air fuel mix.
d. Chirp
• Compression, blowing-by the fire-ring of a damaged head gasket will sometimes produce a
B T
ASIC ROUBLESHOOTING
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