FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
During external timing of a magneto to a reciprocating engine, the technician positions the crankshaft at the manufacturer's specified number of degrees before top dead center on the compression stroke of the No. 1 cylinder. What must occur at exactly this crankshaft position for the magneto to be correctly timed?
- AThe breaker points must just begin to open at the magneto's E-gap position
- BThe magneto rotor must be at full north pole alignment with the coil
- CThe distributor finger must be pointing at the No. 4 cylinder terminal
- DThe impulse coupling must engage and release the flyweights
Show answer & explanationAnswer & explanation
Correct answer: A. The breaker points must just begin to open at the magneto's E-gap position
External timing aligns the engine crankshaft position (specified degrees BTDC) with the exact instant the breaker points open while the magneto rotor is at the E-gap position, ensuring maximum spark intensity occurs at the correct piston position. The distributor points to the correct cylinder terminal separately from this timing event, and the impulse coupling only affects cranking-speed timing, not the basic timing setup.
Why the other options are wrong
- B. Full flux alignment (neutral position) is not the point-opening position; E-gap is slightly past it.
- C. Distributor alignment to the correct cylinder is checked separately, not the timing point itself.
- D. Impulse coupling operation relates to cranking, not to the static timing procedure.
Magneto-to-Engine (External) Timing
The process of setting the crankshaft to a specified degree BTDC and rotating the magneto so the breaker points open exactly at that instant, at the E-gap position.
- Uses a timing light/buzz box to detect point opening
- Crankshaft positioned per engine manufacturer's data
- Points must open exactly at E-gap for max spark energy
- Different from internal magneto timing (rotor-to-points relationship)
Memory trick: Points pop open right when piston hits its mark