Ignition timing is the measurement of the point when spark plugs fire in each engine cylinder. This measurement is taken in the form of degrees of crankshaft rotation, with zero being top dead center (TDC) on the compression stroke of the piston. Timing lights are used to see the degree marks on the crankshaft pulley, and flash when the selected cylinder fires. To explain the general mechanics of setting the timing, a rear wheel drive 1975 Buick Century with a 350 engine is used here to illustrate this process.
Start the engine and let it warm up to a normal operating temperature. When the engine is warm, turn it off.
Attach the timing light to the number one cylinder spark plug, which is at the left front of the engine.
Use a clean rag to clean off the timing marks on the crankshaft pulley. Mark the pulley and timing scale with white chalk. This will make it easier to read when using the timing light.
Disconnect the vacuum hose from the distributor. Plug the hose with a short screw or pencil. This will prevent the distributor from advancing.
Start the engine, and aim the light at the timing marks. For this particular year and engine, the timing should be at 19 degrees before top dead center (BTDC). If the timing is off, it will have to be adjusted.
Turn the car off. Loosen the base clamp on the distributor with the distributor wrenches from GM. Gently turn the distributor to adjust the timing. If the timing needs to be slowed, then turn the distributor clockwise. To speed up the timing, turn the distributor counterclockwise. The direction of rotation depends on the type of engine in the car. Tighten the distributor down.
Start the engine again and recheck the timing. Reconnect the vacuum hose.
