Checking for vacuum leaks on a classic car

Question:

I just purchased a 1948 Ford Deluxe and while driving it home it started backfiring and missing, then it just quit. After about 30 minutes it started up again and was fine for a few more miles, and then it started backfiring again. When I was out on the highway at 55 to 60 mph it seemed to do OK. It was only when I stopped for lights and stop signs that it had problems. I have been told it is a vacuum leak. I think perhaps new plugs and points will help but how do you check for a vacuum leak?

Answer:

To check for a vacuum leak, you will need a vacuum gauge, available at most auto supply stores for a few dollars. Hook up the gauge to any source of vacuum on the intake manifold or carburetor throttle body. (Usually there is a fitting that you can take out, and then you can install a small nipple in its place.) Start your engine and warm it up. At sea level the gauge should read between 18 and 22 inches of vacuum with the engine idling. If the needle reads below normal, that indicates an air leak, in all probability at the intake manifold gasket or the carburetor throttle body gasket.

In Fig. 1 we see that an engine that’s in good condition should indicate a steady 18 to 22 inches of vacuum pressure at sea level with the engine idling. On the other hand, if the needle fluctuates below normal (Fig. 2) with the engine idling, it could indicate an air leak at the intake manifold or its gasket, or at the carburetor gasket. It also could indicate a leaking head gasket.

A regular, intermittent drop below normal (Fig. 3) indicates a leaking valve. Most likely it will be an exhaust valve that is adjusted too tight or perhaps burned. Readjust the valves and check the vacuum again.

A rapid intermittent dropping from normal reading (Fig. 4) indicates sticking valves or dirty hydraulic lifters. Next, try vacuum readings with an open and closed throttle. If you see fluctuations increasing with engine speed (Fig. 5) the problem is weak valve springs.

If, with the engine idling, fast vibrations of normal vacuum are evident (Fig. 6) it is an indication of ignition trouble. On the other hand, slow movement at normal vacuum indicates incorrect carburetor adjustment. And if, when you run the rpm up to 2000 you see a gradual decrease in vacuum reading, check for exhaust system restrictions such as a kinked tail pipe or partially blocked muffler.