Having your timing set to the proper specification will help to ensure the best performance and gas mileage from your 400-cubic-inch Chevrolet small-block. Having your timing "retarded" causes the spark plugs to fire later than they should, which will result in poor performance and gas mileage. Timing too far "advanced" causes the spark plugs to fire early, which can result in pinging under acceleration and can damage the engine. No matter which small-block Chevrolet engine you have, the timing procedure will be the same.
Park the vehicle on a level, paved surface and set the parking brake. Start the engine and allow it to reach normal operating temperature. When operating temperature is reached, turn off the engine.
Remove the rubber vacuum hose from the advance unit on the distributor. Insert the tip of a pencil into the end of the hose to plug it.
Look for the timing marker attached to the timing cover on the driver's side of the engine. It sits just above the harmonic balancer at the 1 o'clock position and is marked in a clockwise direction with numbers representing degrees before and after top-dead-center. Common marks are "Before 18-12-8-4-0-4-8 After." Spray the marker with parts cleaner and wipe it off with a shop rag to make the numbers easier to read. The common timing settings for the 400 small-block are 4 degrees before top-dead-center for automatic transmissions and 8 degrees top-dead-center for manual transmissions.
Find the timing mark on the harmonic balancer so you will know what to look for when you use the timing light. It's a notch in the surface of the balancer. If you don't see it, turn the bolt in the center of the crankshaft with a ratchet and socket in a clockwise direction.
With a wrench, loosen the distributor hold-down bolt just enough so that you can turn the distributor but it takes some effort.
Hook up the timing light. The large pickup lead with the plastic clip is attached to the number-one spark plug wire, which is the first wire on the driver's side of the engine. The red power lead is clicked to the positive battery terminal and the black one to the negative battery terminal.
Start the engine and shine the timing light down at the timing marker. You will see the notch in the balancer line up with a number on the marker. If necessary, turn the distributor to make the notch move to the appropriate number.
Turn off the engine, remove the timing light and reconnect the vacuum advance hose.
