How to Time a Chevy Small Block Motorby Don Bowman
To establish the correct advance curve, the vacuum advance and mechanical advance in the distributor must work properly with no binding. When a small-block Chevy is started, the spark is retarded to 4 degrees BTDC. As soon as the engine starts and a vacuum is produced, the vacuum advance mechanism advances the spark to about 18 degrees BTDC. When the rpm is raised to 2500, the mechanical advance takes over and increases the advance to 32 degrees BTDC. The timing is set at 4 degrees initial to allow the engine to turn over much easier when starting. This is easier on the starter.
Lift the vehicle's hood and remove the air cleaner. Connect the inductive timing light by connecting the red lead to the battery positive post and the black lead to the battery negative or ground. Connect the inductive pickup to the number 1 cylinder spark plug wire. This is the driver's side front spark plug wire on a small-bock Chevy motor. Make sure all the wires are secure and clear of any sharp surface and away from the exhaust manifold.
Start the engine and allow it to warm up. Pull the vacuum line off the vacuum advance on the distributor and plug the hose with a screw. Loosen the distributor hold down nut using a wrench. Tighten the nut just enough to hold the distributor from turning freely, yet allowing you to turn it with your hand.
Shine the timing light carefully at the timing plate on the driver's side of the timing chain cover. The deep V cut in the timing plate is top dead center. The lines and numbers on the top of the plate are degrees before top dead center (BTDC). If the timing light being used is not adjustable, look at the line on the harmonic balancer to see what number on the plate it is lined up with. The target is 4 degrees BTDC on the plate. It will say BTDC on the plate followed by the numbers. If the line is on 8-degrees BTDC for instance, the timing should be retarded to 4-degrees BTDC. Advancing would be 9 or 10 degrees and so on. The Chevy rotor turns clockwise so to advance the spark the distributor must be rotated toward the rotor, or counter-clockwise. To retard the spark, the distributor must be turned clockwise.
Loosen the distributor hold down nut a little more so the distributor can be turned. Make small movements when turning. Rotate the distributor clockwise to retard the spark slightly, lightly tighten the nut, and check where the timing is with the timing light. Repeat until the target 4 degrees BTDC is achieved.
Remove the screw from the vacuum hose and push the hose back on the distributor vacuum advance.
Check the timing again. The vacuum advance should have increased the advance to approximately 18 degrees BTDC plus or minus two degrees. It is as close as you are going to get with an inexpensive distributor.
Hold the rpm at 3000 just long enough to check the timing on the plate. The target is 32 degrees BTDC. If the advance is a little high, rotate the distributor clockwise to drop the advance and conversely counter-clockwise to raise. Once set, shut the engine off and tighten the distributor lock down nut securely.
- Don Bowman, A.S.E. Certified Mechanic
- The more the timing of spark is advanced, the sooner the fuel is ignited in the cylinder. Fuel burns at a specific rate, which does not significantly change with the engine rpm. The fuel is ignited at approximately 18 degrees before top dead center on the compression stroke at an idle. This ignites the fuel early enough so that the fuel is burnt as much as possible before the end of the power stroke. As the rpm increases the time the fuel is in the compression stroke decreases dramatically. If it were still ignited at 18 degrees BTDC, it would not have time to burn completely during the power stroke, so the fuel is ignited earlier to compensate for the shorter time between strokes of the engine.
Things You'll Need
- Inductive timing light
Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).