Engine Testing With A Vacuum Gauge

Vacuum readings at idle much lower than normal can indicate leakage through intake manifold gaskets, manifold-to-carburetor gaskets, vacuum brakes or the vacuum modulator. Low readings could also be very late valve timing or worn piston rings. See figure 3.

Starting with the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, an excessive exhaust back pressure is probably present.

With the engine Idling, the vacuum gauge pointer will drop sharply, every time the leak occurs. The drop will be from the steady reading shown by the pointer to a reading of 10" to 12" Hg or less. If the leak Is between two cylinders, the drop will be much greater. You can determine the location of the leak by compression tests. See figure 4.

All other systems in an engine must be functioning properly before you check the fuel control system as a cause for poor engine performance. If the pointer has a slow floating motion of 4 to 5 inches - you should check the fuel control.