How to Adjust Vacuum Advance Timing

by Michelle Schaefer
itstillruns article image
verchromter motorblock image by Matthias Nordmeyer from

The correct adjustment of a vacuum advance unit is critical to ensure that an automobile engine runs at its peak performance. The vacuum advance mechanism allows the distributor to vary the spark timing to compensate for the load and speed requirements of the engine. During light loads on the engine, the cylinders burn leaner and require the spark to be initiated earlier in order to obtain optimum power. The vacuum advance unit provides this varying spark advance and improves the fuel economy and throttle response throughout the operating range of the engine.

Step 1

Set the mechanical timing of the engine. Disconnect and plug the vacuum advance hose at the distributor and plug the hose. Connect the timing light inductive pickup to the No. 1 spark plug wire and connect the power leads to the vehicle battery.

Step 2

Loosen the distributor hold-down bolt. Start the engine and allow it to warm up to normal operating temperature.

Step 3

Turn the distributor and use the timing light to set the timing to the vehicle manufacturer's specifications. Most cars have a decal in the engine compartment that shows the timing specifications. Tighten the distributor bolt when all adjustments have been made. Rev the engine up to 2,000 rpm and have your assistant observe the timing reading on the harmonic balancer. Record the timing setting. Remove the vacuum advance hose plug and reconnect the hose to the distributor.

Step 4

Remove the vacuum advance hose at the carburetor. Connect the long-hosed vacuum gauge to the carburetor's vacuum advance port. Place the vacuum gauge inside the passenger compartment of the car in a location where it can be read by your assistant when the vehicle is in motion. Ensure the vacuum gauge hose does not interfere with any operating parts of the engine.

Step 5

Drive the vehicle and have your assistant or helper note the maximum amount of vacuum shown on the gauge when the engine is operated at normal highway speeds.

Step 6

Stop the vehicle safely, apply the parking brake and turn off the engine. Connect the timing light inductive pickup to the No. 1 spark plug wire and connect the power leads to the vehicle battery.

Step 7

Connect the vacuum pump with gauge to the distributor vacuum advance. Start the engine and have your assistant rev the motor up to 2,000 rpm. Pump the vacuum up to the reading recorded during the driving test in Step 5.

Step 8

Check the timing mark on the harmonic balancer with the timing light. Record the total advance displayed on the scale. Subtract from this reading the value recorded in Step 3. For example: If the reading in Step 3 was 35 degrees, and the reading from Step 8 is 42 degrees, then 42 minus 35 equals 7.

Step 9

Disconnect the vacuum pump and insert a 3/32 Allen wrench into the port on the vacuum advance where the hose connects. There is a small adjusting screw in the vacuum advance. Turn the screw clockwise to decrease the vacuum advance and counterclockwise to increase the advance. Repeat steps 8 and 9 until the total difference between the mechanical advance and vacuum advance readings is approximately 10 degrees apart. The vacuum advance is now set for 10 degrees of advance at normal highway operating conditions.

Step 10

Remove the vacuum pump and timing light. Reconnect the vacuum-advance hose to the carburetor.

More Articles

article divider