I was thinking about this the other day. Is there any if the vehicle isn't moving? Cause the only thing I know creating a vacuum is the downdraft tube. Anyone put on an exhaust vacuum system on one of these? This is a good question.
The term "engine vacuum" refers to the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. The pistons in the engine serve as suction pumps that create this pressure difference.
Normal Engine Operation: 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.
Find a vacuum port on your carburetor or intake manifold and check to insure that you have steady vacuum at the stated levels. If so, your vacuum brake booster will operate properly.
If the vacuum is not steady, the gauge needle behavior tells a story of engine condition. There are charts available to interpret your gauge readings.