1978 Models (Vacuum Control Valve Test)

Connect tachometer to engine. Start and warm engine to normal operating temperature. Stop engine and remove air cleaner.
On GLC, disconnect and plug vacuum hose from anti-afterburn valve. Disconnect vacuum tube going from vacuum control valve to intake manifold, at manifold, and connect a vacuum gauge to it. See Fig 2.

Fig 1: 1977 Vacuum Control Valve Test
Courtesy of MAZDA MOTORS CORP.
On Federal B1800, disconnect vacuum hose going from air by-pass valve to intake manifold (at manifold). On California B1800, disconnect vacuum hose going from anti-afterburn valve to intake manifold (at manifold). On all B1800 models, connect vacuum gauge to intake manifold. See Fig 2.
On all models, start engine. Increase engine speed to 3000 RPM and quickly release throttle. Vacuum gauge highest reading should be obtained immediately after releasing throttle, drop to 22-23 in. Hg, and then slowly drop to idle vacuum.
If vacuum reading is incorrect, turn adjusting screw in vacuum control valve. Decrease vacuum reading by turning screw clockwise; increase by turning screw counterclockwise.
NOTE: When checking/adjusting vacuum, be sure to correct for altitude. Vacuum gauge readings will be about 1 in. Hg less for every 1000 feet of altitude.

Fig 2: 1978 Vacuum Control Valve Test
Courtesy of MAZDA MOTORS CORP.