ASE A1 Engine RepairGeneral Engine DiagnosisHard
A technician is performing a static ignition timing check on an older vehicle equipped with a distributor. The specification calls for 10 degrees Before Top Dead Center (BTDC) at idle. Using a timing light, the technician observes the timing mark align with 5 degrees After Top Dead Center (ATDC). What adjustment should the technician make?
- AIncrease engine RPM to activate vacuum advance.
- BRotate the distributor clockwise to advance timing.
- CRotate the distributor counter-clockwise to retard timing.
- DReplace the timing chain.
Show answer & explanationAnswer & explanation
Correct answer: B. Rotate the distributor clockwise to advance timing.
The current timing is 5 degrees ATDC, which is very retarded compared to the specification of 10 degrees BTDC. To advance the timing (make it fire sooner, closer to BTDC), the distributor must be rotated clockwise. Each degree of rotation represents a small change in timing.
Why the other options are wrong
- A. Increasing engine RPM activates vacuum or centrifugal advance, which is for dynamic timing, not for setting static base timing.
- C. Rotating the distributor counter-clockwise would retard the ignition timing further (move it further ATDC), worsening the problem.
- D. Replacing the timing chain is a major engine repair and is not indicated by incorrect static ignition timing, unless there's evidence of significant chain stretch, which would typically cause much larger timing discrepancies and other symptoms.
Static Ignition Timing Adjustment (Distributor)
Static ignition timing is the precise moment the spark plug fires relative to piston position (BTDC/ATDC) at idle, adjusted by rotating the distributor.
- Clockwise distributor rotation advances timing (earlier spark).
- Counter-clockwise distributor rotation retards timing (later spark).
- Set using a timing light with engine at specified idle speed, often with vacuum advance disconnected.
Memory trick: Clockwise advances, counter-clockwise delays, timing's set in engine's maze.