On-Vehicle Service (M4Mef) - GMC G Service Manual

6C1-28 CARBURETORS

ON-VEHICLE SERVICE (M4MEF)

Important
m
• Refer to the charts on "Diagnosis of 1MEF and
M4MEF Carburetors" earlier in this section.
FLOAT LEVEL —
EXTERNAL CHECK
Tool Required:
J 34935-1 External Float Level Gage
1. With the engine idling and the choke valve wide open,
insert J 34935-1 in the vent slot or vent hole
(figure 28).
• Allow the gage to float freely.
NOTICE: Do not press down on the gage. Flood­
ing or float damage could result.
2. Observe the mark on the gage that lines up with the
top of the casting. The specified float level setting is
10 mm (13/32-inch). Refer to "Specifications" at the
end of this section.
• Incorrect fuel pressure will adversely affect the
fuel level.
3. If level is more than ± 1.588 mm (2/32-inch) from
specification, remove the air horn and adjust the float.
Figure 28 — Float Level • — External Check
CHOKE CHECKING PROCEDURE —
(ENGINE OFF)
1. Remove air cleaner assembly.
2. Hold throttle halfway open.
3. Open and close the choke valve several times. Be
sure all links are connected and are not damaged.
Choke valve, linkage, and fast idle cam must operate
freely.
• If the choke valve, linkage, or fast idle cam is
sticking due to varnish, clean with Choke
Cleaner X-20-A or equivalent.
• Do not lubricate linkage, as lubricant will collect
dust and cause sticking.
CHECKING ELECTRIC CHOKE — ON-VEHICLE
1. Allow the choke stat to stabilize at about 21 °C (70°F).
2. Open the throttle to allow the choke valve to close.
3. Start the engine. Determine the length of time in
which the choke valve reaches full open position:
• If longer than five minutes, check the voltage at
the choke stat connector, with the engine
running:
• If voltage is between 12 and 15 volts —
check for proper ground between choke
cover and choke housing. If correct,
replace choke cover assembly.
• If voltage is low or zero — check all wires
and connections. Refer to ENGINE
ELECTRICAL (SEC. 6D).
VACUUM BREAK CHECKING PROCEDURE (Figure 29)
Tool Required:
J 23738-A Hand Operated Vacuum Pump
1. If the vacuum break has an air bleed hole, plug it
during this checking procedure (figure 29).
A. Pump Cup or Valve Stem Seal
B. Tape Hole in Tube
C. Tape End of Cover
Figure 29 — Plugging Vacuum Break Bleed Holes
2. Apply 51 kPa (15 in. Hg) vacuum to the vacuum
break.
• Apply finger pressure to the plunger to see if it
has moved through full travel. If not, replace the
vacuum break.
• Observe the vacuum gage. Vacuum should
hold for at least twenty seconds. If not, replace
the vacuum break.
3. Replace vacuum break hoses that are cracked, cut or
hardened.
CHECKING IDLE STOP SOLENOID (ISS)
A non-functioning idle stop solenoid (ISS) (if equipped)
could cause stalling or rough idle.
B-06210