A pilot flying an IO-360-equipped aircraft has encountered an inconsistent and mildly concerning pattern during runup magneto checks: a right-mag drop of 50-60 rpm (well within normal limits) paired with a left-mag drop exceeding 125 rpm, a spread that would ordinarily prompt grounding the aircraft for maintenance. The pilot's workaround—leaning the mixture for a minute or two to "clear" the plugs—brings the left-mag drop down to roughly 100 rpm, putting both the individual drop and the mag-to-mag spread back within tolerance. The engine runs smoothly on each mag independently, and the aircraft passed a pre-buy inspection recently, with a ferry pilot previously attributing the same symptom to slightly fouled plugs. The core dilemma is procedural as much as mechanical: the pilot has been running the check at 1700 rpm with a lean mixture at 5,000 feet density altitude, while Lycoming's Service Instruction 1132B specifies 50-65% power and full rich mixture for the test, raising the question of whether the observed anomaly is a real ignition or plug-fouling issue or simply an artifact of running the check incorrectly.
This scenario sits squarely at the intersection of two things every piston pilot should take seriously: standardized runup procedure and the diagnostic value of magneto checks as an early-warning system for ignition problems. Lycoming SI1132B exists precisely because generic "1700 rpm, watch the drop" runup habits passed down through flight training don't always match the power settings needed to properly load the engine and clear plug deposits before the check is meaningful. Running the check lean-of-normal at altitude, as this pilot has been doing, changes the combustion characteristics enough that comparing the result against POH tolerances calibrated for sea-level, full-mixture runups is not an apples-to-apples comparison. The fact that leaning temporarily "fixes" the drop is itself diagnostically significant—it strongly suggests fouling (carbon or lead deposits on a plug or plugs) rather than a failing magneto, impulse coupling, or internal timing fault, since a genuine mag-drop problem would not be mixture-sensitive in this way. That said, a spread this large, even if repeatable and explainable, is exactly the kind of trend data an owner or operator should be tracking rather than rationalizing away flight after flight.
For working pilots, particularly those flying older piston singles and light twins in Part 91 or flight-training environments, the episode is a useful reminder that runup checks are not pass/fail rituals to rush through but a genuine diagnostic tool, and that the numbers only mean what the reference procedure says they mean if the test is performed the way the reference procedure specifies. A mechanic and an A&P with IA authority would want to know the plug gap history, whether one specific cylinder or plug is chronically fouling, spark plug lead resistance, and impulse coupling condition on the affected magneto before signing off on continued operation. The 200-hours-since-last-500-hour-inspection detail matters too: magnetos are wear-prone components with internal points, cams, and coils that degrade gradually, and a spread that has only become noticeable in the last three flights, after not showing up during a pre-buy, indicates a developing condition worth tracking with logbook entries and possibly an early magneto internal inspection rather than waiting for the next scheduled 500-hour check.
More broadly, this reflects a pattern seen across general aviation with legacy engines still running on leaded avgas and often flown by owners with imperfect access to maintenance support, especially outside major metro areas. The 45-minute drive to the nearest A&P is not an unusual constraint for GA owners, and it creates real pressure to "fly through" an ambiguous symptom rather than ground the airplane, which is precisely the kind of normalization-of-deviance thinking that safety programs like the FAA's GA Safety Enhancements and AOPA's Air Safety Institute repeatedly warn against. The prudent course—running the test exactly per Lycoming's published procedure at the specified power setting and full mixture, documenting results across several flights, and getting a mechanic's eyes on the plugs and mag timing before the anomaly becomes a genuine in-flight ignition event—is the standard any professional operator would apply to a fleet aircraft, and it is worth applying with equal discipline in owner-flown GA, where deferred maintenance decisions carry the same physics-based consequences regardless of certificate type.