How to Troubleshoot an HEI Vacuum Advance

Timing Issues

• HEI vacuum advances are designed to increase or decrease air flow amounts based on how hard the engine is working. In turn, they properly control air/fuel density and create delays that differ from partial throttle to full throttle. But these vacuum advances are often combined with mechanical advances that work using weights instead of air flow. Check both components when troubleshooting. Sometimes mechanical advances are adjusted without taking into account the additional delay provided by the vacuum advance, or the mechanical advance may have developed problems that are affecting the performance of both parts.

Connection Issues

• The vacuum advance is connected to the ignition system via hoses. These hoses may wear out or become disconnected. Check for connection issues and see if you need to refasten the hoses, replace end connectors or replace the hose itself. Even a small amount of extra air can render the vacuum advance useless, since it works by creating air vacuums based on the performance of the engine. Problems may also be caused if you have replaced the distributor or other nearby components but not realigned the vacuum advance.

Clogging Problems

• Dirt and debris can sometimes pile up in the vacuum advance. This not only causes combustion quality issues but also prevents a proper vacuum from forming, lowering the efficiency of the entire system. Dismantling the HEI vacuum advance will allow you to spot any clogging problems. Remove the clogs, clean the system and reassemble. Fortunately, vacuum advance components do not usually require delicate adjustments, so assembly is relatively easy (although aftermarket adjustment components can be added).

Diaphragm Problems

• The diaphragm is the main body of the HEI vacuum advance, the portion of the component that rotates a breaker plate to control air flow and therefore the strength of the vacuum. In older HEI vacuum advances, the diaphragm may be subject to cracks or corrosion. There is no easy solution for these problems. Replace the vacuum advance entirely.