Hi Bob,
The vacuum advance comes from a "ported vacuum" source. This means that
the vacuum takeoff point on the carb is slightly downstream (outboard)
of the throttle butterfly. There is no vacuum on this port with the
butterflies closed or near closed at idle. As the butterflies are
opened, the vacuum port is exposed to engine vacuum to advance the
distributor. The end of the vacuum advance unit should be marked with
three numbers. A typical advance might be marked 7-18-12. The first
number, 7, is the amount of vacuum, in inches of HG, that is needed to
start the advance. The second number, 18, is the amount of vacuum needed
to fully advance the unit. The third number, 12, is the maximum advance
of the unit in distributor degrees. Engine degrees would be twice this
number, or 24 degrees.
You can test the advance unit with the engine running & viewing the
timing marks with a timing light. Disconnect the vacuum line at the
distributor & apply vacuum to the advance unit. A hand pump such as the
"Mityvac" works well. The timing light should show some advance starting
at 7 inches & progressing to 24 degrees at 18 inches of vacuum. Your
actual advance unit may have different numbers marked on it which should
be followed. The vacuum & thus the vacuum advance might be fairly high a
light throttle cruising conditions & will decrease as engine load is
increased to maximum. It will likely be zero at full throttle
acceleration & near zero at maximum car speeds in top gear. It is
possible to change the vacuum unit's calibration by changing the
internal spring tension & the length of the travel limiting sleeve
inside the unit. This is not often necessary unless one is obsessed with
the very maximum fuel economy. Vacuum advance increases throttle
response & gas mileage at lighter throttle settings. It has no effect at
full throttle & is often omitted on race cars.
With the vacuum line still disconnected, observe the timing light as you
increase rpm. Centrifugal advance numbers are typically stated in
distributor degrees & would show twice these numbers on the timing
light. Typical advance numbers as seen on the timing light might be
zero degrees at 500 engine rpm, 15 degrees at 2000 engine rpm, & 26
degrees at 4,500 engine rpm. Please NOTE that all of these numbers would
be in addition to the static advance which might be set at something
like 10 degrees at below 500 engine rpm. Or static plus centrifugal
advance would show 10 degrees at 500 rpm, 25 degrees at 2,000 rpm, & 36
degrees at 4,500 rpm. The most IMPORTANT setting is that the initial
(static) advance plus centrifugal advance should not exceed about 34 to
38 degrees at high engine rpm. No matter what the distributor calibration.
The centrifugal advance calibration is mainly controlled by the two
return springs & the mechanical advance stop. The underside of the
advance unit is often marked in distributor degrees for the maximum
advance that the stop is designed for. In the above example, it would be
marked 13 which is the distributor degrees, 26 in engine degrees.
The two advance springs are usually visually different. The primary
spring will be constructed of smaller wire, less tension, & engages the
advance fly weights from the beginning of travel. The secondary spring
will have some free play in its mounting & engages only when the
distributor has advanced a bit. The two springs together control the
advance from this point to maximum. The object is to have a fairly rapid
initial advance to maybe 2,000 rpm & then more gradual advance to 4,500
rpm. Two stage advance. Spring weights, lengths, & the stop can be
altered to give any desired advance calibration, which is what the
factory does to match the engine's designed advance requirements.
Some play in the distributor drive shaft is not usually a problem. The
mechanical resistance of the distributor tends to keep the free play
always to the loaded side of the gears which effectively removes the
free play. If you observe the timing light position moving more than a
couple of degrees at a constant rpm, the distributor cam assembly may be
moving laterally on it's shaft due to wear on the inside of the cam
assembly or the outside of it's spindle.
Again - Note that the numbers I have quoted are hypothetical & you must
go by the actual numbers for your specific parts.
Dave Russell
BN2
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