Chevrolet 70 1969 Series, Shop Manual

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ENGINE FUEL SYSTEM 6M -9
which is located in the air horn, applies manifold
vacuum within the carburetor and draws a rich
flow of fuel from discharge nozzles of the main
metering systems. When engine starts a spring
arrangement at choke shaft lever permits choke
plate to partially open and prevent excessive
flooding. Choke button must be pushed in to the
position at which engine runs smoothly.
The fast idle mechanism on all models oper­
ates in conjunction with choke.
BENDIX-STROMBERG CARBURETOR ADJUSTMENTS
N O T E : The Bendix-Stromberg (W W C Series)
carburetor (fig. 9) is used on the 401M and 478M
engines.
Correct adjustment of idle speed, ignition
timing, and fuel mixture are extremely important
to ensure proper operation of the vehicle and for
maximum engine performance. A tachometer is
necessary for making the settings as a specific
engine rpm is necessary to obtain correct adjust­
ments.
P R E L IM IN A R Y C H E C K S
1. Thoroughly warm up engine. If the engine
is cold, allow to run for at least 15 minutes.
2. Be sure the carburetor is properly secured
to the intake manifold, which will exclude the possi­
bility of air leaks.
3. Inspect the manifold heat control valve (if
used) for freedom of action and correct spring
tension.
4. If the positive crankcase ventilator valve
should become clogged, the engine idle will be
adversely affected. Therefore, if the engine idle
becomes too slow or rough the ventilator valve
should be checked before any carburetor adjust­
ments are made to compensate for the troubles.
ID L E A N D M IX T U R E A D J U S T M E N T
The idle and mixture adjustments should be
performed with the engine at operating temperature
and choke "open."
Air conditioning (when used) must be turned
on during the carburetor adjustments.
Engines Not Equipped With Controlled
Combustion System (C.C.S.)
1. Check the dwell reading. The dwell reading
for all V6 engines should be 31 to 34 degrees.
2. Set the ignition timing to 10° B U D C with
vacuum advance disconnected and plugged.
3. Set the engine idle speed by adjusting the
idle speed screw (fig. 9). Idle speed is:
V6 Engine (401M and 478M) . . . . 550 Rpm
N O T E : If necessary, back out the fast idle
screw to allow idle speed screw to contact its
stop.
4. Adjust the idle mixture screws (fig. 9).
With engine running and idling speed set, turn
each idle mixture screw in until engine begins to
slow down or run unevenly, then back out screws
until engine runs smoothly with highest vacuum
gauge reading.
N O T E : If vacuum gauge is not available, adjust
idle mixture screws to give highest idle speed.
Turning idle mixture screws "in" gives leaner
mixture; while turning screws "out" gives richer
mixture, D O N O T turn screws in tight against seats
since screws or seats will be damaged.
5. Check idle speed and readjust if necessary,
as described in Step 3.
Engines Equipped with Controlled
Combustion System (C.C.S.)
N O T E : All V6 gasoline engines sold in the
State of California are equipped with C.C .S.
1. Check the dwell reading. The dwell reading
on all V6 engines should be 31 to 34 degrees.
2. Set the ignition timing as indicated by the
"Engine Data Chart” with vacuum advance discon­
nected and plugged.
3. Disconnect and plug vacuum line to vacuum
idle stop (fig. 15).
N O T E : Detailed explanation of vacuum idle
stop is given later in this section under "Controlled
Combustion System."
4. With engine running, vacuum idle stop dis­
connected (line plugged), check reduced engine
rpm. The reduced engine speed should be 350 rpm.
If necessary, readjust the idle speed screw (fig.
15) to obtain 350 rpm.
5. Connect vacuum line from carburetor to
vacuum idle stop. With engine running, vacuum
idle stop connected, engine idle speed should be
550 rpm. If necessary to readjust idle speed per­
form the following:
a. Referring to figure 10, place a %-inch open
end wrench on diaphragm extension of vacuum idle
stop to prevent its rotation while adjusting plunger.
C A U T IO N : U N D E R NO C O N D IT IO N S
S H O U L D T H E D IA P H R A G M E X T E N S IO N
B E A L L O W E D TO R O T A T E O R IN T E R N ­
A L D A M A G E W IL L R E S U L T TO V A C U U M
ID L E S T O P U N IT.
CHEVROLET SERIES 70 -80 H EAVY DUTY TRUCK SH O P M A N U A L