Idle Mixture Adjustment (Heavy-Duty Emission Vehicles Only) - GMC G Service Manual

6C1-30 CARBURETORS
7. Vacuum hoses.
• Refer to the Vehicle Emission Control Informa­
tion Label for vacuum hose routing information.
8. Electrical connectors to the choke and idle stop
solenoid.
9. Air cleaner assembly with gasket.
10. Negative battery terminal.
\^\
Adjust
• Idle mixture and idle speeds.
IDLE MIXTURE ADJUSTMENT
(HEAVY-DUTY EMISSION
VEHICLES ONLY)
In case of a major carburetor overhaul, throttle body
replacement, or high idle CO (when indicated by an emis­
sions inspection), the idle mixture may be adjusted. Adjust­
ing the mixture by other than the following method may
violate Federal and/or state laws:
Figure 32 — Idle Speed Adjustment — Without Idle Speed Solenoid
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 fuel from the carburetor into a container. Dis­
pose of the fuel in an approved container.
4. Remove idle mixture needle plugs (figure 31).
• Invert the carburetor and support it to avoid
damaging 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 degree
angle and drive it into the throttle body until the
casting breaks away, exposing the plug.
• Use a center punch to break the plug apart,
uncovering the idle mixture needle.
• Remove all loose pieces of plug.
• Repeat the previous steps for the other needle
plug.
5. Use J 29030-B to lightly seat the idle mixture needles,
then back out three turns.
6. Re-install carburetor on the engine.
7. Place transmission in "P" (PARK) (automatic trans­
mission) or "N" (NEUTRAL) (manual transmission).
8. Start the engine and bring it to a normal operating
temperature, with the choke valve open, and air con­
ditioning OFF.
9. Connect a known accurate tachometer to the engine.
10. Check ignition timing, and adjust if necessary, by fol­
lowing the procedure described on the Vehicle Emis­
sion Control Information label.