Carburetor Adjustments - GMC G Service Manual

6C1-20 CARBURETORS
reduces the strain on the starting motor and bat­
tery and reduces the possibility of backfiring
while attempting to start the engine. Operate
the throttle several times and check the dis­
charge from pump jets before installing the
carburetor.
3. Carburetor attaching nuts.
H O Tighten
• Both nuts to 4.1 N»m (36 in. lbs.) then both nuts
to 22 N*m (16 ft. lbs.).
4. Accelerator linkage.
5. Electrical connectors for the choke and idle solenoid.
6. Fuel and vacuum lines. Refer to the Vehicle Emission
Control Label, if necessary, for vacuum line routing
information.
7. Air cleaner.
\<ft\ Adjust
• Idle speed. Refer to "Idle Speed Adjustment" in this
section.

CARBURETOR ADJUSTMENTS

A carburetor is designed to meet the particular require­
ments of the engine, transmission and vehicle and although
carburetors may look alike, they are not usually inter­
changeable. Refer to carburetor part number and/or specifi­
cations when making adjustments.
Before suspecting the carburetor as the cause of poor
engine performance or rough idle, check the ignition sys­
tem — including distributor, timing, spark plugs and wires.
Check the air cleaner, evaporative emission system, EFE
System, PCV system, EGR valve and engine compression.
Also inspect intake manifold, vacuum hoses and connec­
tions for leaks, and check torques of carburetor mounting
bolts/nuts.
Make all adjustments with the engine at normal operat­
ing temperature, choke full open, and the air cleaner
installed. Except as noted, air conditioning should be OFF
and all vacuum lines and all electrical leads connected.
Refer to "Specifications" at the end of this section for carbu­
retor adjustment specifications. Gages and tools for making
adjustments are included in the Universal Gage Set
J 9789D.
IDLE MIXTURE ADJUSTMENT (Figure 12)
In the case of a major carburetor overhaul, throttle body
replacement, or high idle CO as indicated by an emissions
inspection, the idle mixture may be adjusted. Adjusting the
mixture by other than the following method may violate
Federal and/or state laws.
Tool Required:
J 29030-B Idle Mixture Needle Socket
1. Set parking brake and block drive wheels.
2. Remove the carburetor from the engine.
3. Drain the fuel from the carburetor into a container.
4. Remove the idle mixture needle plug (figure 12).
• Invert carburetor, and support it to avoid dam­
aging external components.
• Make two parallel hacksaw cuts in the throttle
body, on either side of the locator point near the
idle mixture needle plug. The distance between
the cuts depends on the size of the punch to be
used.
• Cut down to the plug, but not more than
1/8-inch beyond the locator point.
• Place a flat punch at a point near the ends of the
saw marks. Hold the punch at a 45° angle, and
drive it into the throttle body to break the casting
away, exposing the plug.
• Use a center punch to break the plug apart, to
uncover the idle mixture needle.
• Remove all loose pieces of plug.
5. Reinstall the carburetor on the engine.
6. Place the transmission in PARK (automatic transmis­
sion) or NEUTRAL (manual transmission).
7. Start the engine and bring it to a normal operating
temperature, with the choke valve open, and the air
conditioning OFF.
8. Connect an ACCURATE tachometer to the engine.
9. Check the ignition timing, and adjust if necessary, by
following the procedure described on the Vehicle
Emission Control Information label.
10. Use J 29030-B to turn the mixture needle (1/8 turn at
a time), in or out, to obtain the highest RPM (Best
Idle).