GMC 4000 Series Service Manual page 345

Sec. 6M
Page 334
FUEL SYSTEM— GASOLINE ENGINES
Figure 2 3 — Adjusting Vacuum Idle Stop
4. Lack of Power - Lack of engine power that
is isolated to the carburetor can usually be cor­
rected by proper carburetor calibration and ad­
justment. Carburetors should be repaired rather
than replaced when a lack of power complaint is
encountered.
BENDIX-STROMBERG CARBURETOR
ADJUSTMENTS
Correct adjustment of idle speed, ignition tim ­
ing, and fuel mixture are extremely important to
ensure proper operation of the system 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.
Refer to "Engine Tune-up Specifications" in
GASOLINE ENGINES (SEC. 6A) of this manual for
complete tune-up specifications.
PRELIMINARY CHECKS
1. Thoroughly warm-up the engine. If the en­
gine 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 pos­
sibility of air leaks.
3. Inspect the manifold heat control valve (if
used) for freedom of action and correct spring
tension.
IDLE AND MIXTURE ADJUSTMENT
The idle and mixture adjustments should be
performed with the engine at operating temperature,
G M C S E R V I C E M A N U A L
and choke open.
during the carburetor adjustments.
Engines Not Equipped With Controlled
Combustion System - Modified (C.C.S.-M.)
for all V6 engines should be 31 to 34 degrees.
"Engine Data Chart," then reconnect vacuum ad­
vance line.
ing the idle speed screw (figs. 21 and 22).
the fast idle screw to allow idle speed screw to
contact its stop.
and 22). 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 screw s
until engine runs smoothly with highest vacuum
gauge reading.
just idle mixture screw s to give highest idle speed.
Turning idle mixture screw s "in" gives leaner
mixture; while turning screw s "out" gives richer
mixture. DO NOT turn screw s in tight against seats
since screw s or seats will be damaged.
to 550 rpm.
Engines Equipped With Controlled
Combustion System - Modified (C.C.S.-M.)
State of California, covered by this manual, are
equipped with C.C.C.-M.
on all V6 engines should be 31 to 34 degrees.
"Engine Data Chart," then reconnect vacuum ad­
vance line.
idle stop (figs. 30 and 31).
stop is given later in this section under "Controlled
Combustion System."
connected, check reduced engine rpm. The reduced
engine speed should be 350 rpm. If necessary, re­
adjust the idle speed screw (figs. 21 ans 22) to ob­
tain 350 rpm.
vacuum idle stop. With engine running, vacuum idle
stop connected, engine idle speed should be 550
rpm. If necessary to readjust idle speed perform
the following:
end wrench on diaphragm extension of vacuum idle
stop to prevent its rotation while adjusting plunger.
Air conditioning (when used) must be turned on
1. Check the dwell reading. The dwell reading
2. Set the ignition timing as indicated by the
3. Set the engine to run at 550 rpm by adjust­
NOTE: On 351M engine, if necessary, back out
4. Adjust the idle mixture screw s (figs. 21
NOTE: If vacuum gauge is not available, ad­
5. Check idle speed and re-adjust if necessary
NOTE: All V6 gasoline engines sold in the
1. Check the dwell reading. The dwell reading
2. Set the ignition timing as indicated by the
3. Disconnect and plug vacuum line to vacuum
NOTE: Detailed explanation of vacuum idle
4. With engine running, vacuum idle stop dis­
5. Connect vacuum line from carburetor to
a. Referring to figure 23, place a %-inch open