Large magneto drop after 100-hour inspection
A four-cylinder Lycoming O-360 in a training aircraft has just completed a 100-hour inspection. The spark plugs were cleaned, gapped to 0.016-0.019 in, and reinstalled; the magneto-to-engine timing was checked and found correct; the ignition harness was visually inspected. On run-up at 1800 rpm, the engine runs smoothly on both magnetos. Selecting the right magneto produces a 175 rpm drop with noticeable roughness; selecting the left produces a 55 rpm drop. At 700 rpm idle, the engine is rough on both magnetos and smoothest after leaning. The pilot reports no in-flight roughness at cruise power. The right magneto is a newly overhauled exchange unit installed 30 hours ago; the left is original. All four bottom spark plugs are oily; the top plugs are light grey. Oil consumption is 1 US qt per 6 hours and the engine has 1,650 hours since major overhaul.
1. What does the difference between the right and left magneto drops tell you, and what is the most likely cause?
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The left magneto has an acceptable drop and the right magneto has an excessive drop with roughness. That localizes the fault to the right ignition circuit, but it does not prove the right magneto itself is faulty. The key clue is that the oily plugs are all bottom plugs, and in a typical dual-ignition installation one magneto fires the bottom plugs while the other fires the top plugs. If the right magneto fires the oil-fouled bottom plugs, its drop will be large and rough, while the left magneto firing the clean top plugs will look normal.
At cruise power the higher cylinder pressure and mixture strength can fire across a fouled plug or run acceptably on one plug, which explains the lack of in-flight roughness. At idle the mixture is richer and the fouled plug short-circuits more easily, so both-magneto operation is rough. A newly overhauled magneto with correct timing is less likely to be internally faulty, but a damaged harness lead or cap is still possible.
2. How would you isolate the fault without unnecessary parts replacement?
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I would first check the right magneto harness for chafing, cracked insulation, loose terminal nuts and correct lead placement, then test the spark plugs or swap the top and bottom plugs between ignition circuits. If the excessive drop follows the oily bottom plugs to the other magneto, the fault is plug fouling or an oil-control problem, not the magneto. If the right magneto still has the excessive drop with known-good plugs and leads, the magneto or its internal timing is suspect.
I would also perform a differential compression check and inspect the cylinders with a borescope, because oil-fouled bottom plugs at 1 qt per 6 hours suggest oil passing the rings or valve guides. Cleaning the plugs may restore a normal drop temporarily, but recurrent oil fouling is not fixed by cleaning. A magneto change would be wasted money if the root cause is oil entering the combustion chamber.
3. What is your airworthiness decision if cleaning the plugs restores a 50 rpm drop but the bottom plugs oil up again after 5 hours?
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I would not return the aircraft to unrestricted service. A 50 rpm drop after cleaning shows the ignition circuit can perform, but oil fouling recurring in 5 hours is abnormal and will eventually cause misfire, roughness and possible pre-ignition. The engine is consuming oil at 1 qt per 6 hours, which supports a mechanical oil-control problem rather than a plug problem.
I would investigate the affected cylinders: compression check, borescope of cylinder walls and valve guides, and inspection of ring condition and valve-guide clearance. Depending on findings, the cylinder may need repair or replacement. I would document the recurring fouling, the compression and borescope results, and the corrective action. If the operator wants a deferral, it must be within the manufacturer's and regulator's allowed limits, with a defined inspection interval; a recurring 5-hour fouling pattern would not meet a normal release standard.