GMC 4000 Series Service Manual page 416

Figure 7— Checking Breaker Arm Spring Tension
"Specifications," at end of this section.
6. Install rotor, position, and secure distrib­
utor cap to housing.
SETTING DWELL ANGLE
The point opening of new points can be checked
with a feeler gauge, but the use of a feeler gauge
on rough or uncleaned used points is not recom­
mended since accurate mechanical gauging cannot
be done on such points.
Contact points must be set to the proper open­
ing. Points set too close may tend to burn and pit
rapidly. Points with excessive separation tend to
cause a weak spark at high speed. Proper point
setting for these engines are listed in "Specifica­
tions" at end of this section.
New points should be set to the larger opening
as the rubbing block will wear down slightly while
seating to the cam. Contact points should be clean­
ed before adjusting if they have been in service.
CONTACT POINT OPENING ADJUSTMENT
1. Release distributor cap hold-down screw s,
remove cap and place it out of work area.
2. If necessary, align points by bending the
fixed contact support. Do not bend the breaker
lever. Do not attempt to align used points; replace
them where serious misalignment is observed.
Use an aligning tool if available.
3. Turn the distributor shaft until the breaker
arm rubbing block is on the high point of the cam
lobe. This will provide maximum point opening.
4. Loosen the contact support lock screw.
5. Use a screwdriver to move point support to
obtain a 0.019" opening for new points (or 0.016"
opening for used points) (fig. 8).
6. Tighten the contact support lock screw and
recheck the point opening.
G M C S E R V I C E M A N U A L
point opening to specifications, the cam angle or
dwell should be checked with a dwell meter, if such
equipment is available (see "Specifications" at end
of section for proper dwell angle).
LINE WHEN CHECKING. If the cam angle is less
than the specified minimum, check for defective
or misaligned contact points or worn distributor
cam lobes. The variation in cam angle readings
between idle speed and 1750 engine rpm should not
exceed 3 degrees. E xcessive variation in this speed
range indicates wear in the distributor.
above 1750 engine rpm may prove unreliable on
some cam angle m eters.
spark advance mechanism located under the break­
er plate assembly. The advance mechanism is part
of the main shaft assembly and consists of an auto­
matic cam actuated by two centrifugal weights
controlled by springs.
advance, based on engine speed, is necessary to
develop maximum power. As engine speed increas­
es, the weights are gradually thrown out against
the springs and rotate the cam to provide the de­
sired spark advance for speed at which engine is
running.
calibration have been selected to provide spark
advance which will give the best performance of
engine throughout its entire speed range. The
IGNITION SYSTEM
Figure 8 — Setting Point O p ening
7. After checking and adjusting the contact
NOTE: DISCONNECT AND PLUG VACUUM
NOTE Cam angle readings taken at speeds
CENTRIFUGAL A D VAN C E
Distributor is equipped with a centrifugal
Where speed variations are encountered, spark
The correct weights, cam contour, and spring
Sec. 6Y
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