Testing Vacuum Advance Distributors on Classic Cars

Muscle car vacuum advance distributor
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, then I'm sure you have seen a vacuum advance distributor. When these somewhat antiquated systems are working properly, these cars are pleasures to drive.

However, when they malfunction, the once mighty muscle car develops an annoying stumble or lack of power under part throttle conditions. Here we'll talk about some of the common problems associated with a vacuum advance distributor and provide some straightforward testing procedures to help you verify the cause before buying replacement parts.

Symptoms of Deteriorating Vacuum Diaphragms

When it comes to our aging classic car's rubber components are some of the first to deteriorate. Whether you're talking about manually operated fuel pumps 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, when the diaphragm becomes compromised, it will drip raw fuel on the ground through a weep hole to notify the driver.

On an automatic transmission if a vacuum modulator leaks the engine pulls the transmission fluid into the combustion chamber, creating an abundant amount of smoke in conjunction with transmission shifting problems. The heart of a vacuum advance for the distributor is a rubber diaphragm that converts applied vacuum into advanced timing. When this diaphragm begins to deteriorate the symptoms can start off slow, which makes them hard to notice.

Eventually, when it deteriorates to the point where it no longer adjusts the timing, the automobile will develop a hesitation when the engine attempts to move the vehicle's weight. In addition to this lack of power, the small vacuum leak can also cause a rough engine idle and possibly a stalling condition. If you do find it necessary to replace your vacuum advance on the distributor, 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 like to use an inductive pickup timing light and verify the base timing is set to the correct position first. Then they can connect a hand-operated vacuum pump to the diaphragm, give it a few pumps and watch the timing mark on the crankshaft harmonic balancer advance on the timing scale with the timing light. Nevertheless, those without ignition timing lights can still test the vacuum advance.

With the engine off you can remove the distributor cap and use a vacuum hand pump to operate the advance mechanism. The diaphragm moves a rod that in turn moves a sliding plate at the base of the distributor and you can see this movement with the naked eye. A few pumps of a manually operated vacuum tester should not only provide full advance, but it should stay in that position until you remove the vacuum from the port.

Additional Vacuum Advanced Testing

If your cars, vacuum advance is working properly and holding negative pressure, there could 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 and some will use manifold vacuum. Both types are controlled in the throttle position and provide an accurate determination of engine load.

Manifold vacuum is highest with the throttle plates closed and the vehicle is idling. At this time ported vacuum is nonexistent. They create a ported signal when the throttle plates 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.