How To Adjust Vacuum Advance On HEI Distributor: Everything You Need To Know About Vacuum Advance On HEI Distributor

The vacuum advance is a separate system, and it uses a small diaphragm assembly connected to the distributor side. At part throttle, the diaphragm gets moved in the vacuum advance canister on the distributor by a high manifold vacuum. As a result, more timing gets added. However, at WOT, the vacuum drops to near zero. Therefore, the vacuum advanced gets removed, and the initial plus the mechanical advance establishes the total timing.

Vacuum advance is entirely independent of mechanical advance. The vacuum advance is described as load-based timing because timing is added when the engine has a light load.  Conversely, the timing will reduce when there is more load in the machine.

More people are starting to use HEI distributors in their engines. Having an excess vacuum advance can lead to engine knock. Therefore to prevent this from happening, you will have to reduce your vacuum advance. Adjusting your vacuum advance is a simple task, and it will take you a little time to finish the process. The following is all you will have to know about the vacuum advance and HEI installation.

How To Adjust Vacuum Advance On HEI Distributor

The vacuum advance adjustment is necessary when spark knock occurs under part throttle. You can adjust your vacuum advance by inserting the 3/32 inch Allen wrench into the vacuum advance canister port. Once you have inserted the Allen wrench, you will then turn it two turns counter-clockwise. After the adjustments, you will test-drive the car.

The HEI Distributor Installation

Most people nowadays use the High Energy Ignition distributor on their General Motors. You should ensure that before you start the installation, you read all the instructions first. To install the HEI distributor, you will have to remove your original distributor.

Before removing your original distributor, ensure you check the base timing and note the position of the vacuum advance canister. Moreover, you should also note the rotor position and have new HEI-style spark plug wires.

How do you check the base timing? The first thing you will do is bringing the car engine to its usual temperatures. You will then disconnect and connect the vacuum hose connected to the vacuum advance canister. Once you have connected it tightly, you will start the car engine and check the base timing.

Distributor Removal

before you install your HEI  distributor, you first need to remove your original distributor. Here are the steps you can follow to remove your original distributor successfully.

  • The first thing is to disconnect the negative battery cable. Once the negative battery cable gets disconnected, you will disconnect the vehicle wiring harness. You will also disconnect the distributor wiring harness from the distributor cap.
  • You will then remove the distributor cap while ensuring you leave the spark plug wires attached. Once you do that, set the distributor cap and the wires out of the way.
  • You will then observe and memorize the distributor rotor position. Moreover, ensure you label the wires connected to the coil terminals and then disconnect them.
  • Once you have disconnected the wires attached to the coil terminals, remove the oil and coil bracket from the engine.
  • The next task is removing the vacuum hose from the vacuum canister. Once you have removed the vacuum hose, you can now remove the distributor.
  • You should ensure you carefully lift the distributor on the distributor housing while slightly turning the rotor.
  • After removing the distributor, check the distributor gear for any potential problems.

Distributor Installation

Once you have withdrawn your original distributor, you can now install your HEI distributor. You can follow the following procedures to ensure you have a successful installation.

  • The first step is disconnecting the wire harness from the distributor cap and removing it from the distributor housing.
  • What will follow is installing the distributor gasket over the gear and up to the distributor collar. Ensure that you lubricate the distributor gear.
  • The next thing is to ensure the distributor shaft gets turned until the rotor is in the same position before removing the original distributor.
  • You will then place the distributor into the engine. When putting the distributor, the rotor will start turning as it engages with the camshaft gear. As it is rotating, make adjustments to the rotations by turning the rotor a few degrees before gear engagement. You should ensure that the distributor collar gets placed entirely flat on the intake manifold.
  • Turn the distributor housing until the vacuum advance canister is in the exact direction before removing the original distributor.
  • The next task is installing the distributor hold down and tightening the hold-down bolt slightly. You will then tighten the hold-down bolt thoroughly after the ignition timing adjustment and install the new distributor cap.
  • You will then move the spark plug wires one by one from the original distributor cap to the new one. Once you have done that, connect the wire harness and vehicle wiring into the distributor cap.
  • If converting from a point-type ignition, ensure you remove resistance wires or ballast resistors previously used in the original distributor.
  • The next thing is reconnecting the battery and plugging the vacuum hose attaching to the vacuum advance canister. Ensure you confirm that all spark plug wires are secure and in their correct positions.
  • You will then start the engine and check the base timing. You will ensure you adjust your timing until you achieve the required timing.
  • After achieving the correct timing, switch off the ignition and tighten the distributor hold down.
  • The last thing is to re-attach the vacuum hose to the vacuum advance canister. Once you follow these steps to the latter, you can be sure to install your distributor successfully.

How To Adjust Your Vacuum Advance On The HEI Distributor

You will know that you have too much vacuum advance when running lean while cruising and not accelerating. The maximum amount of vacuum advance should be 14 degrees. If it is past that, then you will have excess vacuum advance.

Too much vacuum advance will cause the engine to knock or ping. Therefore when you notice there is too much vacuum advance, you will need to adjust it. Here are the steps you can follow to change your vacuum advance.

  • The first thing is assembling the tools to perform the adjustments. The device you will need is the Allen key. It would be best to consider using the Allen key that has T-handles. The reason for this is that it offers better control and provides more comfort while using it.
  • You will then disconnect the vacuum line from the canister. You do not have to run your engine when making the adjustments. Therefore, you will not have to worry about plugging the line after removal.
  • Enter the Allen key into the vacuum nipple and change the amount of vacuum advance. The vacuum advance system works by clockwise and anti-clockwise rotations. When you rotate the Allen key clockwise, you will be reducing the vacuum advance. If you turn it anti-clockwise, you will be increasing the vacuum advance.
  • Therefore, in this case, you will rotate the Allen key clockwise to reduce the excess vacuum advance.
  • The last thing you will do is to test and repeat. The process involves taking the car for a test drive. If you find that the engine is still knocking, you will repeat the same process until it stops.

How To Monitor Your Vacuum Advance Adjustments

Once you have made the adjustments,  you will have to keep reading them and monitoring them to ensure they are in order. The tools you will need for your monitoring are a timing light and a timing tab with some paint or timing tape. You will connect your timing light to the battery and the number one cylinder’s sparkplug.

Once they get hooked, you will point the timing light on the harmonic balancer to take your reading. The timing mark on your harmonic balancer will line up with the timing tab to let you know when the spark plugs are firing. When the engine speed increases, ensure that you watch that timing mark with the tab to determine your ignition timing level.

When Should Vacuum Advance Start?

You can start the process of vacuum advance when your engine idles at ten inches HG or less. You will begin by setting the initial timing to at least 15 degrees BTDC. You may pull the manifold vacuum down even further by placing your car equipped with automatic transmission in gear.

How Do You Know that Your Vacuum Advance Is Bad?

There are signs to show your vacuum advance is terrible. Poor gas mileage and poor throttle response are some of the signals to let you know that your vacuum advance is defective. Therefore ensure you are keen to not these signs.                    


Having a curve in the distributor together with a vacuum advance is a good thing. It helps in reducing low-speed timing where the engine might detonate with too much timing. The vacuum advance is better than the mechanical advance because it offers more excellent fuel economy and engine performance.

You can adjust the vacuum advance with an Allan key by either rotating it clockwise or anti-clockwise. The clockwise rotation will reduce the excess vacuum advance, while the anti-clockwise turning will increase the vacuum advance.

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David Huner
David Huner
David Huner is a tech lover. After completing his graduation from the University Of Phoenix, he started gather his knowledge mostly on latest technologies that keeps his life smart and cool. Now he wants to spread his knowledge with people who loves technologies.

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