MTD P90 series Shop Manual page 26

P90 Series Vertical Shaft Engines
chirping noise.
• Confirm with a compression test and leak-down test.
e. Unusual exhaust tone
Splashy or blatty
• Splashy idle usually indicates a slight rich condition.
• May indicate an exhaust blockage, usually slightly muffled.
Backfire
• On over-run: unburned fuel igniting past exhaust valve. Mixture not burning completely in com-
bustion chamber. It may be too rich or it may be spark-plug or ignition problem.
• Occasional, under load: engine momentarily runs lean, usually will cycle with float bowl level or
governor pull-in, sometimes sounds like a slight stumble. Ethanol content exceeding 10% will
make the engine run artificially lean.
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• Usually ignition related.
• Run the engine with a spark tester in-line between the spark plug wire and the spark plug or
use an oscilloscope and see if the spark goes away at the same time the engine dies.
4. Engine over-speed
A. Continual over-speed
• Binding or damaged external governor linkage or carburetor throttle.
• Mis-adjusted governor arm.
• Internal governor failure.
B. Momentary over-speed
• Intermittent bind (very unusual).
• Interference: This is fairly common when debris can fall on the governor linkage during normal
operations.
5. Engine RPMs surge (hunting)
A. Over-governed condition- Return spring replaced with wrong part or hooked into wrong hole.
NOTE: This is an extremely rare condition, usually created by tampering.
B. Lean Air-fuel mixture condition- When AFR (Air Fuel Ratio) is significantly below stoichiometric ratio
(14.7:1) engine RPMs sink until they reach a point that can be supported by the available fuel. This
causes a momentary surge in power until the available fuel is consumed, then the RPMs fall again,
repeating the cycle.
• Too much air: look for an air leak in the intake tract
• Not enough fuel: look for fuel supply or carburetor problems
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