Diagnosis Of Power Problems - GMC G Service Manual

DIAGNOSIS OF POWER PROBLEMS

PROBLEM
1. (1MEF)
Engine Hesitates
During Normal
Acceleration
'Hesitates During
Cold Engine
Operation
2. ' Choke not operating properly.
3. ' Vacuum break assembly(s) malfunctioning
4. 'Thermae system malfunctioning.
5. *EFE system malfunction.
6. Ignition timing misadjusted.
7. Distributor vacuum or mechanical advance
8. Carburetor float level too low.
9. (1MEF)
ENGINE. PRIVEABILITY AND DIAGNOSIS 6A-11
POSSIBLE CAUSE
Malfunctioning carburetor pump system.
Make a quick check of the pump system:
With the engine off, look into the carburetor
bore and observe pump discharge jet; actu­
ate throttle lever. A full stream of fuel should
emit from the jet.
(M4MEF)
Malfunctioning carburetor pump system or
misadjusted pump.
Make a quick check of the pump system:
With the engine off, look into the carburetor
bore and observe pump discharge jets; actu­
ate throttle lever. A full stream of fuel should
emit from the jets.
or misadjusted.
malfunctioning.
Carburetor power piston sticking. Missing or
damaged power piston spring, power piston
rod, power rod link, or pump and power rod
lever.
(M4MEF)
Carburetor power piston sticking. Missing or
damaged power piston spring.
CORRECTION
1. (1MEF)
Remove air horn and check pump cup. If
cracked or hardened, replace the pump
plunger cup and spring.
Inspect pump well for scoring. Replace
float bowl if necessary.
Check for restricted pump passages.
Clean and blow out passages with com­
pressed air.
Check the pump discharge ball for proper
seating.
(M4MEF)
Remove air horn and check pump cup. If
cracked or hardened, replace the pump
plunger cup and spring.
Check for restricted pump passages.
Clean and blow out passages with com­
pressed air.
Check the pump discharge ball for proper
seating.
Adjust pump to specifications.
2. Check for complete opening and closing of
choke valve. See "Choke Checking Pro­
cedure (Engine Off)" and "Checking Electric
Choke" in CARBURETORS (SEC. 6C1).
3. See "Vacuum Break Checking Procedure" in
CARBURETORS (SEC. 6C1). Adjust vac­
uum break(s) to specification.
4. Check operation. Refer to the "Fuel and
Emissions Service Manual" and CAR­
BURETOR EMISSIONS (SEC. 6E1).
5. Check operation. Refer to the "Fuel and
Emission Service Manual" and CAR­
BURETOR EMISSIONS (SEC. 6E1).
6. Adjust timing. See Emission Control Informa­
tion Label on vehicle.
7. Check operation. Refer to ENGINE ELEC­
TRICAL (SEC. 6D).
8. Check float level, and adjust if necessary.
9. (1MEF)
Check power piston operation: push piston
down against piston rod and release; it
should move freely in bore and return to its up
position. Rotate throttle to wide open; piston
should move upward in bore.
If piston does not move freely, clean power
piston and bore.
If piston does not return when depressed,
or does not move upward when throttle is
opened, check for missing or damaged
parts. Replace as necessary.
(M4MEF)
Check power piston operation: push piston
down and release; it should move freely in
bore and return to its up position.
If piston does not move freely, clean power
piston and bore.
If piston does not return when released,
check for missing or damaged power piston
spring. Replace as necessary.
GMTB-0910-2L