9.6 Ignition and Starting Troubleshooting

Spark, Timing, Fuel, and Cranking Speed

A reciprocating engine starts and runs only when it has compression, the correct fuel-air mixture, a strong spark, and correct timing. The starting system must rotate the engine fast enough for induction, compression, and ignition to occur. The ignition system must create spark at the correct time. The fuel system must deliver a combustible mixture. Because these requirements overlap, a no-start question is solved by sorting which requirement is missing and why.

Magnetos are engine-driven generators for ignition. Once rotating, a magneto can make spark without battery power. That independence improves reliability but creates a maintenance hazard: a magneto with a broken P-lead ground path may remain live even when the cockpit switch says off. During a magneto check, selecting one magneto grounds the other. If there is no RPM drop when one side is selected off, the supposedly grounded magneto may still be firing. That is a safety and airworthiness issue, not a normal result.

Timing controls when combustion pressure rises. If spark occurs too early, pressure can oppose piston travel and cause kickback or detonation risk. If spark occurs too late, power falls and exhaust temperature may rise because combustion continues too far into the exhaust stroke. Impulse couplings or other starting aids retard spark and increase spark energy during cranking. A failed impulse coupling can make starting hard, create kickback, or leave the engine without the expected start assist.

Spark plug condition is a common cause of roughness. Lead, carbon, oil, cracked insulation, incorrect gap, and worn electrodes reduce spark quality or let spark leak to ground. Harness defects can do the same. A roughness that changes when selecting left or right magneto helps isolate the side, but it does not automatically prove the magneto itself is bad. The affected side includes magneto, harness, plugs, switch circuit, and timing.

Starting-system troubleshooting should not stop at the starter motor. A starter contactor may click without carrying enough current. A cable can look intact and still have high resistance. A ground strap can heat under load. A starter drive can fail to engage or fail to disengage. Excessive engine drag from oil viscosity, internal damage, or accessory problems can overload an otherwise good starter. Voltage-drop testing under load is more useful than open-circuit guesses.

Use this sequence when an engine will not start:

1. Check cranking speed and starter electrical behavior.
2. Confirm fuel quantity, fuel selection, priming, mixture, and throttle position.
3. Confirm ignition switch position, magneto grounding, starting aid action, and spark quality.
4. Consider timing, induction leaks, flooded condition, vapor, and mechanical compression.
5. After any start abnormality, inspect for starter engagement, kickback damage, fire risk, and maintenance manual limits.

For turbine engines, starting logic shifts toward starter speed, fuel scheduling, ignition excitation, light-off, temperature rise, and starter cutout. A hot start means temperature rose beyond limits because fuel, airflow, acceleration, or start sequence was wrong. A hung start means acceleration stops below self-sustaining speed. The same cause-effect discipline applies: air, fuel, ignition, rotation, and timing must agree.