How To Adjust Vacuum Advance On HEI Manifold - Everything You Need To Know About Vacuum Advance On HEI Manifold (2023)

The vacuum feed is a separate system and uses a small diaphragm assembly that attaches to the side of the manifold. At part load, the membrane in the vacuum tank in the distributor is moved by a high vacuum in the manifold. As a result, more time is added. However, at WOT, the vacuum drops to almost zero. Therefore, the advanced vacuum is removed and the initial plus mechanical advance gives the total time.

The vacuum feed is completely independent of the mechanical feed. Vacuum advance is described as a load dependent time because time is added when the engine is under a light load. On the other hand, the time is shortened when the machine is loaded more heavily.

More and more people are using HEI distributors for their engines. Too high a vacuum advance can lead to engine knocking. Therefore, to prevent this, you must reduce the vacuum advance. Setting the vacuum advance is a simple task and will take a little time to complete. Below is everything you need to know about vacuum feed and HEI installation.

How to adjust the vacuum line on the HEI manifold

(Video) Part 2 The Ultimate Vacuum Advance Tuning Guide

Vacuum line adjustment is required if sparking occurs at part load. You can adjust your vacuum advance by inserting the 3/32" allen wrench into the vacuum advance canister port. After inserting the Allen key, turn it counterclockwise two turns. After hiring you will test drive the car.

Installation des HEI-Distributors

Today most people use the High Energy Ignition distributor for their General Motors. You must ensure that you read all the instructions first before beginning the installation. To install the HEI manifold, you must remove your original manifold.

Before removing the original manifold, be sure to check the base time and note the position of the vacuum forerun bottle. You should also look at the rotor position and have new HEI style ignition wires.

How do you check the base time? The first thing you will do is get your car's engine up to normal temperatures. Then disconnect and connect the vacuum hose that is connected to the vacuum feed bottle. After connecting it securely, start your car engine and check the base time.

(Video) Vacuum Advance - Ported vs Manifold: Which to Use & When !!!

Removal of the distributor

Before installing your HEI manifold, you must first remove your original manifold. Here are the steps you can follow to successfully remove your original distributor.

  • The first thing to do is to disconnect the negative battery cable. After the negative battery cable is disconnected, disconnect the vehicle's electrical harness. You will also disconnect the distributor wire harness from the distributor cap.
  • Next, remove the distributor cap, making sure to leave the spark plug wires connected. Once you've done that, set the distributor cap and wires out of the way.
  • Then observe and note the position of the distributor rotor. Also, be sure to label the wires connected to the coil terminals and then disconnect them.
  • After disconnecting the wires connected to the coil terminals, remove the oil and coil mount from the motor.
  • The next task is to remove the vacuum hose from the vacuum tank. After removing the vacuum hose, you can remove the elbow.
  • Be sure to gently lift the distributor into the distributor housing while rotating the rotor slightly.
  • After removing the transfer case, inspect the transfer case for possible problems.


After removing your original distributor, you can now install your HEI distributor. You can follow the procedures below to ensure a successful installation.

  • The first step is to disconnect the distributor cap wiring harness and remove it from the distributor body.
  • Next the distributor seal is fitted over the gear and up to the distributor collar. Be sure to lubricate the transfer case.
  • Next you need to make sure the distributor shaft is rotated until the rotor is in the same position before removing the original distributor.
  • Next, put the distributor on the engine. As the distributor is placed, the rotor will begin to rotate as it engages the camshaft gear. When spinning, make adjustments to the RPM by rotating the rotor a few degrees before engaging the gear. It is important to ensure that the distributor collar lies completely flat on the intake manifold.
  • Rotate the dispenser body until the vacuum feed canister is in the exact direction before removing the original dispenser.
  • Next, install the manifold gasket and lightly tighten the drain plug. Then fully tighten the set screw after the ignition timing adjustment and install the new distributor cap.
  • Next, move the spark plug wires one at a time from the original distributor cap to the new one. Once you have done this, connect the vehicle's wire harness and wiring to the distributor cap.
  • When converting from a point ignition, it is essential to remove the previously used resistance wires or ballast resistors from the original distributor.
  • Next, reconnect the battery and connect the vacuum hose that connects to the vacuum feed canister. Make sure all spark plug wires are tight and in the correct position.
  • You will then start the engine and check the base time. They make sure to adjust your time until you reach the required time.
  • After the correct time has been reached, switch off the ignition and tighten the distributor.
  • Finally, reconnect the vacuum hose to the vacuum forerun bottle. After completing these steps to the last, you can be sure of a successful installation of your distributor.

How to adjust the vacuum line on the HEI manifold

You'll know you have too much vacuum advance if you're running lean and not revving while riding. The maximum vacuum progression should be 14 degrees. If you go beyond that, you have excessive vacuum feed.

Too much vacuum adjustment will cause the engine to jerk or stutter. So if you find that the vacuum advance is too big, you need to adjust it. Here are steps you can take to change the vacuum lead.

(Video) How to set vacuum advance on older cars

  • First, the tools for the adjustments must be assembled. The device you need is the Allen key. It would be better to use the hex wrench with T handles as it will give you better control and more comfort when using.
  • You will then disconnect the vacuum line from the canister. You don't have to keep the engine running when making adjustments. So you don't have to worry about connecting the line after removing.
  • Insert the Allen key into the vacuum nipple and change the height of the vacuum advance. The vacuum feed system works clockwise and counter-clockwise. Turning the Allen key clockwise decreases the vacuum advance. Turning it counterclockwise increases the vacuum advance.
  • So in this case turn the Allen key clockwise to reduce the vacuum overshoot.
  • The last thing you will do is test and repeat. The process includes a test drive of the car. If you notice the engine is still knocking, repeat the process until it stops.

How to monitor your vacuum feed settings

Once you've made the adjustments, you need to keep reading and monitoring them to make sure they're okay. The tools required for monitoring are a timing light and an ink or time tape timing guide. You connect your timing light to the number one cylinder battery and the spark plug.

Once hooked, aim the timing light at the harmonic balancer to take your reading. The timing mark on your harmonic balancer aligns with the timing guide so you know when the spark plugs will fire. As engine speed increases, be sure to observe the ignition timing mark with the guide to determine ignition timing.

When should the vacuum advance start?

You can start the vacuum adjustment process when the engine is idling at 10 inches HG or less. You start by setting the start time to at least 15 degrees BTDC. You can further reduce manifold vacuum by engaging a gear on your automatic transmission equipped car.

How do you know your vacuum feed is bad?

There are signs that your vacuum growth is terrible. Poor fuel economy and poor throttle response are some of the signs that your vacuum advance is faulty. So make sure you don't look forward to these signs.

(Video) The Vacuum Advance Debate!


A bend in the manifold along with aVacuumAdvance is a good thing. This helps reduce the time at low rpm where the engine can stall for too long. Vacuum advance is better than mechanical advance as it offers better fuel economy and engine performance.

You can adjust the vacuum advance with an Allen key by turning it clockwise or counterclockwise. Clockwise rotation reduces excessive vacuum advance, while counterclockwise rotation increases vacuum advance.

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How do you adjust the vacuum advance on a hei? ›

If spark knock occurs under part throttle, an adjustment to the vacuum advance is necessary. This adjustment is made by inserting the 3/32" Allen wrench into the vacuum advance canister port and turn it two turns counter-clockwise. Test-drive the car and repeat the adjustments until the spark knock is eliminated.

What should vacuum advance be set at? ›

Ideally, the timing now should be somewhere around 34 to 36 degrees to total advance. With 34 degrees of total mechanical advance and 14 degrees initial timing, you have 20 degrees of mechanical advance—14 + 20 = 34.

Should vacuum advance have vacuum at idle? ›

Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn.

How many degrees does vacuum advance add? ›

A vacuum advance unit can add up to 15 degrees of advance. The amount of advance depends on how much vacuum is applied.

How does vacuum advance affect timing? ›

By design, vacuum advances are spring offset to give zero advance. Only when a strong enough vacuum signal is present will it advance the ignition timing. The higher the vacuum signal, the greater the timing advance. High vacuum signals are indicative of low engine loads at a closed or nearly closed throttle position.

What is a normal vacuum reading? ›

At idling speed, an engine at sea level should show a steady vacuum reading between 14 in. and 22 in. Hg. A quick opening and closing of the throttle should cause the vacuum to drop below 5 in., then rebound to 23 in. or more.

How do I know if my timing is too advanced? ›

Some things to look out for:
  1. Engine knocking: Engine knocking occurs when the air-fuel mixture is ignited too soon in the cylinder. ...
  2. Decreased fuel economy: The timing of the spark plug is crucial during the ignition process.
1 Mar 2021

What happens if ignition timing is too advanced? ›

If the spark timing is initiated too early, the cylinder may experience detonation and potentially cause damage. If the spark occurs too late, the engine runs flat, makes less power, and may overheat.

What should manifold vacuum be at idle? ›

Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in. -Hg.

Does vacuum increase with throttle? ›

As the throttle blades open, restriction and vacuum are reduced. This allows more air/fuel into the cylinder and rpm increases. With the throttle wide open, airflow is maximized and vacuum is at its least.

At what pressure does a vacuum start? ›

Vacuum can refer to any pressure between 0 PSIA and 14.7 PSIA and consequently must be further defined. For applications concerned with measuring vacuum pressures over this full range, two different approaches are often taken. Vacuum pressure is measured relative to ambient atmospheric pressure.

Does vacuum advance go to manifold vacuum? ›

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum ...

What is the function of vacuum advancer? ›

The function of a vacuum advance is to detect the rising vacuum of a car engine. The vacuum advance then triggers the ignition in “advance", when the level of vacuum reaches a predetermined set level.

What should spark advance be at idle? ›

In Forscan "Spark advance" is about 3 - 6 degrees and "Ignition timing" is a lot more, about 33-35 degrees. These values are at idle.

Do HEI distributors have vacuum advance? ›

See all 6 photos In a GM HEI distributor, vacuum advance is limited by the length of the advance slot. The further the rod travels, the more spark lead the unit provides. Original GM vacuum advance canisters can provide as much as 25 degrees, and possibly even more.

How do you adjust a vacuum switch? ›

Adjustable Vacuum Switch
  1. Insert a small hex driver into the screw pictured.
  2. Tighten or loosen the screw until you reach the desired set point. Tighten clockwise to increase vacuum; loosen counter-clockwise to decrease vacuum.
20 Jul 2018


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