How to Test the Vacuum Advance in Classic Cars

Muscle car vacuum advance distributor
Mark Gittelman

If you have a classic muscle car like a second generation Dodge Charger or even a General Motors product like the first generation Chevrolet Monte Carlo, you've probably seen a vacuum advance. When this somewhat antiquated system is working properly, the car is a pleasure to drive.

But when the vacuum advance malfunctions, the once-mighty muscle car develops an annoying stumble or lack of power under part throttle conditions. To avoid these types of problems, you'll want to give your vacuum advance regular check-ups to make sure it's in working order.

Symptoms of Deteriorating Vacuum Diaphragms

When it comes to aging cars, rubber components are often the first to deteriorate. Whether you're dealing with a manually operated fuel pump or a vacuum modulator mounted on an automatic transmission, it's a simple fact that a diaphragm will not last forever. In the case of a fuel pump, a compromised diaphragm will drip raw fuel on the ground through a weep hole to notify the driver that there's a problem.

On an automatic transmission, a vacuum modulator leak will cause the engine to pull the transmission fluid into the combustion chamber, creating an abundance of smoke—along with transmission shifting problems. The heart of a vacuum advance is a rubber diaphragm that converts applied vacuum pressure into advanced timing. When this diaphragm begins to deteriorate, the symptoms can be slow to appear, which makes them hard to notice.

Eventually the deterioration will reach the point that the vacuum advance no longer adjusts the timing, causing the vehicle to hesitate when the engine attempts to move the vehicle's weight. In addition to this lack of power, a vacuum leak can also cause the engine to idle roughly or even stall. If you find it necessary to replace the vacuum advance, take the time to check the carburetor choke pull off, as the internal diaphragms are about the same thickness and share roughly the same lifespan.

Testing the Vacuum Advance

There are a couple of ways to test the vacuum advance on vehicles equipped with distributors. Mechanics prefer to use an inductive pickup timing light. Once the base timing is set to the correct position, you can connect a hand-operated vacuum pump to the diaphragm, give it a few pumps, and then watch the timing mark on the crankshaft harmonic balancer advance on the timing scale with the timing light.

The second way to test the vacuum advance does not require a pickup timing light. With the engine off, remove the distributor cap and use a vacuum hand pump to operate the advance mechanism. The diaphragm moves a rod that turns a sliding plate at the base of the distributor, which can be seen with the naked eye. A few pumps of a manually operated vacuum tester should provide a full advance, and the sliding plate should stay in place until you remove the vacuum from the port.

Additional Advanced Testing

Even if your car's vacuum advance is working properly and holding negative pressure, there could still be a problem with the vacuum signal itself. There are two different kinds of vacuum sources provided to the distributor advance diaphragm. Some automobiles use ported vacuum pressure, while others use manifold vacuum pressure. Both types are controlled in the throttle position and provide an accurate determination of the engine load.

Manifold vacuum pressure is highest with the throttle plates are closed and the vehicle is idling. At this time there is no ported vacuum pressure. The throttle plates create a ported signal once they open and air flows past the base plate of the carburetor. Deteriorated or broken vacuum lines can create the same symptoms as a malfunctioning vacuum advance.