Ignition Timing

1. Ignition Timing R. Bortignon

2. What is Ignition Timing? • ignition timing determines when the spark occurs in relation to piston/crankshaft position • ignition timing is stated in degrees of crankshaft rotation • TDC = Top Dead Center • BTDC = Before Top Dead Center • ATDC = After Top Dead Center • ignition timing adjustment must be set correctly for best vehicle performance, fuel economy & lowest emission levels • ignition timing is not the same procedure as valve timing • on vehicles with distributors, ignition timing should be checked & readjusted during tune-ups & after the distributor has been removed & reinstalled from the engine • timing can only be adjusted on vehicles with distributors! • vehicles with DIS or direct ignition has the timing controlled by the computer and is not adjustable

3. How to Check & Adjust Ignition Timing • connect the timing light to the spark plug wire on cylinder #1 • timing light flashes every time #1 spark plug fires • timing specs can be located in the under-hood tune-up decal or in Mitchell/Alldata

4. Follow timing preparation procedures… • this prepares the engine for ignition timing • refer to the under-hood tune-up decal • engine at normal operating temperature • vacuum advance disconnected? • correct idle speed • computer controlled engines • disable computer’s influence on timing

5. Checking & Adjusting the Timing • clean & paint the timing scale & notch (with engine off)

6. Adjusting the Timing • slowly rotate the distributor while the engine is running until the marks are aligned • tighten distributor hold-down bolt then re-check timing