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AC3 Series of Engines
16
•Knock
* Check for loose mounting of engine or driven implement
* Rotate crankshaft back-and-forth to check for loose connecting rod.
• Click
* Half-engine speed clatter: loose valve lash.
* Half-engine speed clatter, slightly heavier: wrist-pin.
* Rhythmic heavy-light engine speed click: piston slap
• 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.
XI. Unusual exhaust tone
a. Splashy, blatty, wheezing or whistling.
• Splashy or blatty idle usually indicates a slightly rich condition.
• Whistling or wheezing may indicate an exhaust blockage, usually slightly muffled.
• Backfire
* On over-run: unburned fuel igniting past exhaust valve. Mixture not burning completely in
combustion chamber. It may be too rich or it may be a spark plug or an ignition problem.
• Skip
* 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.
XII. Engine RPMs surge (hunting)
a. 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