During external timing of a magneto, a technician connects a timing light (buzz box) across the breaker points. What indication from this device tells the technician that the points have just begun to open, corresponding to the E-gap position?
- AThe light flickers continuously regardless of point position, showing only battery condition
- BThe light stays continuously illuminated as long as the points remain closed
- CThe light or buzzer activates at the exact instant the points separate and the circuit opens
- DThe light dims gradually as the points close, then remains dim until the crankshaft is rotated further
Show answer & explanationAnswer & explanation
Correct answer: C. The light or buzzer activates at the exact instant the points separate and the circuit opens
A timing light (buzz box) is connected so that current flows and the light/buzzer is active while the points are closed; when the points open, the circuit breaks and the light goes out (or a buzzer stops), or in some designs it lights the instant the points open. Regardless of wiring convention, the change of state at the exact instant of point separation identifies the true firing position (E-gap), allowing precise magneto-to-engine timing.
Why the other options are wrong
- A. Constant flicker unrelated to points would indicate a faulty setup, not proper timing indication.
- B. Continuous illumination while closed does not by itself identify the opening instant precisely; the change of state does.
- D. Gradual dimming does not correspond to how a timing light circuit operates; the change is essentially instantaneous.
Timing Light (Buzz Box)
A test device connected across magneto breaker points to precisely indicate the instant the points open, used for external magneto-to-engine timing.
- Detects the exact moment of point separation (E-gap position).
- Used with the engine set at the specified degrees BTDC on the correct cylinder.
- Ensures accurate alignment between piston position and spark timing.
Memory trick: Light out, points apart — that's the spark's starting mark.