Vacuum Advance Unit

Vacuum System:

Vacuum circuit control is provided by a thermal vacuum valve. The valve senses engine coolant temperature.

Vacuum System - Coolant Temperature Below 95 Degrees F:

When engine temperature is below 95° F (35° C), thermal vacuum valve connects top with middle port. This causes vacuum advance to be operated by throttle edge port vacuum via vacuum regulator. A delay valve prevents sudden loss or increase of vacuum when throttle valve is moved quickly. The vacuum regulator won't allow vacuum to be applied to the advance unit until the "carb" port receives vacuum signal.

Vacuum System - Coolant Temperature At Or Above 95 Degrees F:

Once engine temperature reaches 95° F (35° C), the thermal vacuum valve connects middle and lower ports. This allows vacuum advance to be controlled through the vacuum regulator.

A vacuum regulator is incorporated to give a better idle condition. The vacuum regulator is located between solenoid vacuum valve and vacuum advance unit. The lower port of vacuum regulator is sourced to throttle butterfly. During idle conditions, atmospheric pressure operates on vacuum regulator limiting vacuum signal available to vacuum advance unit to 11.0 in ± 1 in Hg 9.372 ± .033 bar)

Vacuum Hose Routing:

Dump valve has two vacuum lines. One connects to right intake manifold at bottom of throttle body. This supplies a vacuum signal to dump valve and acts on a diaphragm inside valve. While vacuum signal is present, dump valve is held closed to atmosphere. The other line connects with a tee between vacuum regulator and distributor vacuum unit. When there is sudden throttle opening, vacuum signal disappears in manifold line and that allows the tee line to be vented to atmosphere, causing timing to retard and prevents possibility of ignition detonation.