TM 9-4910-730-14&P
4-27. VACUUM CONTROLLED BREAKER
closing as well as the point of opening of the distrib-
PLATE TEST
utor points. This is an extremely accurate measure-
ment of the points dwell and the actual dwell at each
4-28. The breaker plate must be smooth and even
individual lobe may be determined.
in its travel or the plate will twist, causing a location
change of the cam follower which will cause, in turn,
4-22. CENTRIFUGAL ADVANCE CALIBRA-
a change in dwell angle and consequent deteriora-
TION TEST
tion of the engine performance. The test procedure
is as follows;
4-23. This test is made to determine if the ignition
timing is in conformance with the manufacturers re-
a. Adjust the speed control to approximately 1000
rpm.
quired curve throughout its range of speed. A defec-
tive centrifugal advance will downgrade the perfor-
b. Connect the DWELL test lead to the distribu-
mance of the engine and can cause knocking and
tor.
c. Vary the vacuum regulator to apply a range of 0
overheating.
to 20 inches hg while at the same time watching the
4-24. To check the centrifugal advance mechanism
dwell meter for changes.
proceed as follows;
d. If the dwell reading varies more than 2 degrees
from zero to 20 inches hg it would indicate intermit-
a. Set zero of the degree ring in line with the ar-
row flash most convenient to the operator.
tent distributor resistance or, in the case of distribu-
b. Increase the distributor speed, pausing at each
t o r s which have centrally located breaker plate
specified speed to note if the amount of advance oc-
bearings, worn bushings or bearings. Some distribu-
curring is within the range specified.
tors with side pivoted breaker plates may normally
c. Momentarily exceed the highest speed given as
have a change of more than 2 degrees, refer to the
a test point, and then, while returning the speed
manufacturers specifications in these instances.
towards zero, recheck at each test speed to see that
the advance at that speed is the same as it was in
4-29. VACUUM SPARK ADVANCE TEST
step b. preceding, and is within specifications.
d. If the advance is excessive in both the ascend-
4-30. The vacuum spark advance adjusts the
ing and descending modes of test, the governor
engine timing to provide optimum performance with
s p r i n g s are weak, or the wrong springs are in-
different load conditions. While performing this test
stalled.
the operator should not fail to observe the mechan-
e. If the advance is low in the ascending mode and
ical movements of the distributor mechanism. The
e x c e s s i v e in the descending mode the governor
breaker plate should move smoothly without stick-
weights are sticking and should be freed up.
ing or warping. To check the vacuum advance unit
f. If the advance is insufficient in both modes of
proceed as follows;
test, the governor spring tension is excessive.
a. With the distributor operating at approximate-
ly 200 rpm and no vacuum applied, set the degree
4-25. VACUUM CHAMBER DIAPHRAGM
ring zero in line with one of the arrows.
TEST
b. Refer to the test specifications for the distribu-
tor to obtain the proper speed, vacuum and advance
4-26. The following tests apply to both single and
specifications.
dual diaphragm vacuum advance units. Both the vac-
c. Check the distributor in both ascending (in-
uum advance side and the spark retard side of the
creasing speed) and descending (decreasing speed)
dual units are tested alternately as if they were
modes of operation.
single units. The test procedure is as follows;
d. If the advance is excessive in both the ascend-
a. If necessary install proper adapter in the dis-
ing and descending modes of test, the governor
tributor port.
springs are weak or the wrong springs are installed.
b. Hold the vacuum hose doubled to seal off the
e. If the advance is low in the ascending mode and
vacuum, switch on the vacuum pump and adjust the
e x c e s s i v e in the descending mode the governor
vacuum pump REGULATOR until 15" of vacuum (or
weights are sticking and should be freed up.
the manufacturers specified value) is obtained.
f. If the advance is insufficient in both modes of
c. Connect hose to the distributor port adapter,
test, the governor spring tension is excessive.
the gauge should return to 15" (or other preset
value) in a few seconds. If the gauge fails to return,
4-31. SYNCHRONIZING POINTS ON
the vacuum chamber is leaking and must be re-
DOUBLE BREAKER ARM DISTRIBUTORS
placed.
