Fig. 1: View of the servo diaphragm (arrow), located
on the intake manifold next to the carburetor
Fig. 2: View of the vacuum control valve, located next
to the servo diaphragm and bolted to the intake manifold
Start the engine and set the idle speed to 800 rpm. Stop the engine.
Disconnect the vacuum sensing tube between the servo diaphragm and the vacuum
control valve at the servo diaphragm.
Remove the intake manifold suction hole plug.
Connect the intake manifold and the servo diaphragm with a tube so that
the intake manifold vacuum goes directly to the servo diaphragm.
Connect a tachometer and remove the vacuum sensing tube between the carburetor
and distributor.
Start the engine and read the speed. If the engine is running between 1,300–1,500
rpm, the servo diaphragm is operating normally. If the engine speed is 800–1,500
rpm, adjust the speed with the throttle opening screw. If the engine speed
remains normal, about 800 rpm, the servo diaphragm is defective and should
be replaced.
