Vacuum advance unit for a 1980 Mazda 626
The pictures below show a vacuum advance unit
for early model (rear wheel drive) Mazda 626's, from
1979 to 1982.
The photo at the bottom shows the vacuum advance unit
mounted on the distributor of a 1980 Mazda 626.
The main purpose of the vacuum advance unit is to advance
the ignition timing of the engine at throttle openings above
idling.
A hose is connected from the vacuum chamber at the top of
the unit to the carburettor.
The vacuum present in the throat of the carburettor at medium
throttle openings, causes the diaphragm inside the vacuum advance
unit to move upwards.
The diaphragm has a metal arm (linkage) connected to it, so when
the diaphragm moves upwards, the linkage also moves.
The linkage is connected to the breaker plate in the distributor, and
when it moves upwards, it rotates the breaker plate thereby advancing
the engine ignition timing.
If the accelerator pedal is pressed to the floor, the 'butterfly' valve in
the carburettor fully opens, causing the vacuum in the throat of the
carburettor to drop dramatically, and this in turn allows the diaphragm
in the vacuum advance unit to return to its normal position, causing the
ignition timing to be retarded (temporarily).
The maximum advance of the engine ignition timing achievable with the unit
shown in the photos is around 13 degrees. That's because the linkage arm
has "6.5" stamped on it.
The vacuum advance units for the 1981 & 1982 Mazda 626's are identical
to the one shown here, except they have "9.0" stamped on the linkage
arm, and are therefore capable of advancing the ignition timing by up
to 18 degrees.
Problems that can occur with vacuum advance units are -
* Ruptured, distorted, or perforated diaphragm (most common).
* A seized breaker plate assembly inside the distributor which
will not move when the linkage arm of the vacuum advance
unit attempts to rotate it in accordance with the changing vacuum
conditions in the carburettor.
Both these problems will result in reduced on-road performance
because the ignition timing is not receiving the extra advancement
from the vacuum advance unit.
All photos copyright © 2007 John Barnes.
Vacuum advance unit
Upside-down view of vacuum advance unit showing the linkage arm which
connects to the rotatable breaker plate in the distributor.
Side view of vacuum advance unit showing vacuum chamber at top.
There is a diaphragm inside the unit below the vacuum chamber.
unit mounted on distributor
Showing vacuum advance unit mounted on the side of the distributor
Tools 
