If the under-hood label does not provide a base timing value, what is an acceptable RPM range for checking base timing?

• A. Within 50 RPM
• B. Within 100 RPM of the idle speed specification listed on the under-hood label or in the ECS manual.
• C. Within 200 RPM
• D. Within 10 RPM

Answer: (B)

Base timing is the ignition timing measured at idle. If no exact base timing value is given on the under-hood label, use the engine’s stated idle speed as the benchmark. You want the timing to fall within about 100 RPM of that idle speed specification listed on the label or in the ECS manual. This range accounts for normal measurement and mechanical variations while keeping timing aligned with typical operating conditions. Tighter tolerances would be unrealistically sensitive to small changes, and a wider window could let timing drift enough to affect performance and emissions. So, within 100 RPM of the specified idle speed is the practical, acceptable range.