-Find and disconnect the vacuum line to the distributor. Connect the vacuum gauge to the line.
-START ENGINE, outside of garage, and run until Engine operating temperature is reached and choke is completely off, transmission in Park.
-Determine your vehicles ideal idle speed in RPM; usually 600-800 RPM. If you have a high performance cam it may be as much as 1100 RPM.
-Connect tachometer to determine idle rpm. -Adjust carburetor idle screw to achieve desired rpm.
-TURN ENGINE OFF.
-Connect Timing Light
-START ENGINE and point timing light at mark on damper.
Record the timing reading and the vacuum reading.
-Disconnect the vacuum gauge.
-Re-connect the vacuum line to the distributor.
-Run the engine up to 3000 RPM, point timing light as above. - Record the timing reading.
-TURN ENGINE OFF
You have now established the theoretical .