DIeck the reed contact areas of the valve holder for
grooves, scratches, any signs of separation from the
holder, or heat damage. DIeck the sealing lip coating
on the valve holder for the same signs. If there is any
doubt as to the condition of the reed contact areas or
the sealing lip, replace the air suction valve as an assem-
If any carbon or other foreign particles have accu-
mulated between the reed and the reed contact area,
wash the valve assembly with a high flash-point solvent.
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air suction valv. _mbly replacement.
Although the vacuum switch valve usually permits
secondary air flow, it shuts off the air flow when a
high vacuum (low pressure) is developed at the engine
side of the carburetor bores during engine braking. This
is to prevent explosions in the exhaust ports which
might be caused by extra unburned fuel in the exhaust
during deceleration, if fresh air were injected into the
These explosions or "backfiring" in the
exhaust system could damage the air suction valves.
Regular inspection of the vacuum switch valve is not
If backfiring occurs frequently in the exhaust
system during engine braking or if there are abnormal
engine noises, check the vacuum switch valve as follows:
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screw po.ition I. preset to determine 'pring prelotd.
Turning the screw Mil cause valve malfunction.
certain that all the hoses are routed without being
flattened or kinked, and are connected correctly to the
air cleaner housing, vacuum switch valve,
carburetor holders, and air suction valve covers.
they are not, correct them or replace them
-Warm up the engine
Vacuum Switch Valv. Operation
-Note the frequency and the loudness of backfiring in
the exhaust system which takes place when the throttle
valves are quickly opened and then closed.
engine to about 4,000
Too Iowa speed does not
generate a high enough vacuum to operate the vacuum
switch valve, and too high a speed is not necessary and
may be harmful to the engine.
-Stop the engine.
-Remove the fuel tank (Pg. 46).
-At the air cleaner housing, disconnect the hose which
connects the air cleaner housing and the vacuum switch
-Plug the hose fitting on the air cleaner housing so that
unfiltered air does not enter the air cleaner housing
through the hose fitting.
Plug the hose that is dis-
connected so that no air can flow to the air suction
valves through the vacuum switch valve.
A. Hose Fitting
-Start the engine, and note the frequency and loudness
at the backfiring as before.
-If the backfiring occurs in the same manner
cases, the vacuum switch valve works properly. If the
backfiring is different the second time, the vacuum
switch valve is defective and must be replaced with a
Vacuum switch adjustment is not
To Air Suction
Secondary Air Flows (Valve is opened)
Secondary Air cannot flow during Engine Braking