GMC 1976 ZEO 6083 Maintenance Manual page 366

6M- 24
ENGINE FUEL SYSTEM
14
IDLE MIXTURE ADJUSTMENT
IN THE CASE OF HIGH IDLE CO (OVER .5%) MAJOR CARBURETOR
OVERHAUL, THROTTLE BODY REPLACEMENT, OR WHEN POOR IDLE
QUALITY EXISTS, IDLE MIXTURE MAY BE ADJUSTED . T O PROPERLY SET
IDLE MIXTURE TO ACHIEVE THE SMOOTHEST IDLE WHILE MAINTAINING
EMISSION LEVELS WITHIN THE STANDARDS PRESCRIBED BY FEDERAL
LAW, THE FOLLOWING PROCEDURES MUST BE FOLLOWED :
BEFORE SUSPECTING THE CARBURETOR TO BE THE
CAUSE OF POOR ENGINE PERFORMANCE OR ROUGH IDLE,
CHECK IGNITION INCLUDING DISTRIBUTOR, CHECK TIMING,
PLUGS AND WIRES, CHECK AIR CLEANER, EVAPORATIVE
EMISSION SYSTEM, PCV SYSTEM AND COMPRESSION .
ALSO, CHECK VACUUM HOSES FOR LEAK .
IF ACCURATE CO METER IS AVAILABLE .
1 . WITH ENGINE AT NORMAL OPERATING TEMPERATURE. REMOVE AIR
CLEANER AND DISCONNECT AIR CLEANER VACUUM HOSE AT INTAKE
MANIFOLD, THEN PLUG FITTING .
2 . CHOKE OPEN AIR CONDITIONING OFF.
3 . SET PARKING BRAKE AND BLOCK FRONT DRIVE WHEELS.
4. DISCONNECT CARBURETOR HOSES FROM VAPOR CANISTER,
DISTRIBUTOR, AND PLUG HOSES .
5. CONNECT CO METER TO EXHAUST SYSTEM TAILPIPE .
6. SET IDLE SPEED TO 600 RPM .
ENGINES CERTIFIED FOR 1974-1975-1976
(REFER TO LABEL ON R.H . VALVE COVER FOR YEAR OF CERTIFICATION)
(HOWEVER)
7 . TURN EACH IDLE MIXTURE CAP IN EQUAL AMOUNTS (MAXIMUM
ADJUSTMENT IS ONE HALF) UNTIL IDLE CO IS AT OR BELOW
.2%. RESET IDLE SPEED, IF NECESSARY, WITH AIR CLEANER IN PLACE.
8 . RECONNECT CARBURETOR TO CANISTER HOSE AND DISTRIBUTOR
VACUUM ADVANCE HOSE. INSTALL AIR CLEANER AND CONNECT
VACUUM HOSE .
NOTE : CO METER MUST BE CAPABLE OF LOW LEVEL, ACCURATE
READINGS . METERS SHOULD BE ACCURATE WITHIN ± .1% CO
IN THE SETTING RANGE SPECIFIED . THE METER SCALE SHOULD
HAVE DIVISIONS OF .2% CO OR LESS.
Figure 3 0-Carburetor Adjustments
IF ACCURATE CO METER IS NOT AVAILABLE.
I . WITH ENGINE AT NORMAL OPERATING TEMPERATURE . REMOVE AIR
CLEANER AND DISCONNECT AIR CLEANER VACUUM HOSE AT INTAKE
MANIFOLD, THEN PLUG FITTING,
2 . CHOKE OPEN AIR CONDITIONING OFF.
3 . SET PARKING BRAKE AND BLOCK FRONT DRIVE WHEELS .
4 . DISCONNECT CARBURETOR HOSES FROM VAPOR CANISTER,
DISTRIBUTOR AND PLUG HOSES.
5 . SET DWELL. SET TIMING AT SPECIFIED RPM .
6. BREAK TABS OFF FROM IDLE MIXTURE SCREW CAPS .
7. CONNECT AN ACCURATE VACUUM GAUGE TO THE INTAKE MANIFOLD .
8. WITH TRANSMISSION IN DRIVE, ADJUST IDLE SPEED SCREW TO
OBTAIN 600 RPM.
9 . EQUALLY RICHEN (TURN OUT) MIXTURE SCREWS UNTIL MAXIMUM
IDLE SPEED IS ACHIEVED . NOTE MANIFOLD VACUUM READING .
NOTE : IF MIXTURE SCREWS ARE APPARENTLY OUT-OF-BALANCE
OR CARBURETOR IS BEING OVER-HAULED, LIGHTLY SEAT
IDLE MIXTURE SCREWS, THEN BACK OUT 4 FULL TURNS.
ADJUST IDLE SPEED SCREW TO OBTAIN 650 RPM,
10. EQUALLY LEAN (TURN IN) MIXTURE SCREWS UNTIL THE IDLE SPEED
IS 600 RPM. MANIFOLD VACUUM READING SHOULD NOT BE REDUCED
BY MORE THAN 2 INCHES OF MERCURY FROM READING OBTAINED
IN STEP 9 . IF READING IS REDUCED MORE THAN 2 INCHES,
REPEAT PROCEDURE.
11 . RECONNECT DISTRIBUTOR AND CANISTER HOSES.
12 . INSTALL AIR CLEANER AND AIR CLEANER VACUUM HOSE .
13. IDLE CO SHOULD BE .23{, MAXIMUM.
A-4350