The centrifugal timing is a function of the centrifugal mechanism. If it
is not correct, it can be fixed by cleaning accumulated lubricant and dirt
and repairing worn components. Fine tuning of the centrifugal
component can be accomplished with the tension of the springs
restraining the centrifugal weights. A distributor testing machine is
necessary to accurately check centrifugal advance timing and make all
adjustments.
Vacuum timing is a function of the vacuum advance mechanism and is
also tested using a distributor testing machine. It either performs as
intended or it does not. However, there are adjustable vacuum advance
mechanisms available for some distributors, but not all.
The initial timing is set when an engine tune-up is performed. It is
accomplished by setting the engine position to the specified timing mark
found on the flywheel or crankshaft damper when the No. 1 piston is in
its top dead center position on its compression stroke. Because each
piston makes two complete revolutions for each complete cycle —
intake, compression, power and exhaust — the timing mark can appear
when the piston is not on its compression stroke. The compression stroke
can be determined by holding a thumb over the spark plug hole and
feeling the pressure buildup on the compression stroke. There are also
devices that be can installed in the spark plug hole that whistle on the
compression stroke as air is forced through the device.
With the No. 1 piston in its correct position indicated by the timing
mark on the compression stroke, the initial timing can be set close to the
required specification with the engine stopped by using a test lamp (see
Figure 2). Connect one lead of the test lamp to the distributor primary
terminal and the other to ground. Then, with the ignition switch on,
slightly rotate the distributor clockwise or counterclockwise until the
lamp just lights. If a test lamp is not available, remove the high-tension
lead from the center of the distributor cap. Hold this terminal about ^1 ⁄ 8
inch from the closest ground (the distributor case or block). Again, turn
the distributor clockwise or counterclockwise until a spark jumps from
the high-tension lead to the ground.
The distributor is now set in the firing position at specified idle speeds
and is sufficiently accurate to start the engine. This assumes that the
rotor is pointed to the No. 1 cylinder spark plug wire connection.
Fine-tuning the initial timing requires that the engine be fully warmed