To set the timing on a Chevy 305 engine, locate the timing mark near the harmonic balancer. Turn the crankshaft with a ratchet until the timing notch is visible. Attach a timing light to spark plug one and the battery, disconnect the vacuum advance hose from the distributor, and loosen the distributor hold-down slightly. Adjust the distributor until the timing light illuminates on the 6 degrees before mark on the harmonic balancer, then drive the car to test the timing.
After disconnecting the hose from the vacuum advance, use a screw to plug the hole in the distributor. When attaching the timing light, refer to the timing light instructions to ensure the light is connected to the battery properly. The number one spark plug is typically the first spark plug on the right side of the engine.
Loosen the distributor hold-down only enough to turn the distributor with effort. It is often necessary to adjust the timing slightly after setting it to 6 degrees before top-dead-center.
To adjust the timing, set the timing an additional 2 degrees before top-dead-center. Drive the car after each adjustment. When the car clatters upon acceleration, set the timing back 2 degrees. Reattach the hose on the vacuum advance, and disconnect the timing light after adjusting the timing.