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