Chevrolet 10 Series 1968, Service Manual

Page: 395 / 713

ENGINE FUEL 6M-2
IDLE MIXTURE
IDLE SPEED
F ig . 3 C —Rochester 4M V
F ig . 4 C —H o lley 4150G
SERVICE PROCEDURES
Prelim inary Checks
1. Thoroughly warm-up engine. If the engine is cold,
alow to run for at least 15 minutes.
2. Inspect torque of carburetor to intake manifold bolts
and intake manifold to cylinder head bolts to exclude
the possibility of air leaks.
3. Inspect manifold heat control valve (is used) for
freedom of action and correct spring tension.
Adjust Idle Speed and M ixture
1. As a preliminary adjustment, turn idle mixture
screws lightly to seat and back out 3 turns.
CA U TIO N :
Do not turn idle mixture screw
tightly against seat or damage may result.
2. With engine running at operating temperature (air
cleaner installed, preheater valve and choke valve
wide open); adjust idle speed screw to bring idle
speed to specified rpm (automatic transmission in
drive, manual transmission in neutral).
NOTE:
On air conditioning vehicles, turn air
conditioning "O F F ” except L-6 engines with
automatic transmission where idle speed is set
with air conditioning "O N ” .
3. Adjust idle mixture screw to obtain highest steady
idle speed. Adjust idle speed screw to recommended
rpm on "D EC AL” .
NOTE: Idle speed adjustment for L-6 engines
and Powerglide transmissions with idle solenoid
is as follows: adjust idle speed to 500 rpm by
adjusting solenoid plunger hex only. Disconnect
wire at solenoid and observe idle speed. De­
energizing the solenoid allows the throttle lever
to seat against the carburetor idle screw. Adjust
the carburetor idle screw as necessary to obtain
400 rpm.
4. Adjust mixture screw in to obtain a 20 rpm drop
(lean roll).
5. Adjust mixture screw out 1/4 turns.
6. Repeat Steps 4 and 5 for second mixture screw (if
so equipped).
7. Readjust speed screw if necessary to specified idle
rpm.
Choke Adjustment (Remote Choke) (Fig. 5C )
1. Remove air cleaner and check to see that choke
valve and rod move freely.
2. Disconnect choke rod at choke lever.
3. Check choke adjustment as follows: Hold choke valve
closed, pull choke rod downward against stop. The
top of choke rod should be even with bottom of hole
in choke lever.
4. If necessary, adjust rod length by bending rod at
offset bend. (Bend must be such that rod enters
choke lever hole freely and squarely).
5. Connect rod at choke lever and install air cleaner.
Choke Adjustment (M anual Choke)
1. Remove air cleaner.
2. Push hand choke knob in to within 1/8” of instrument
panel.
3. Loosen choke clamp at carburetor bracket and ad­
just cable through the clip until the choke valve is
wide open.
4. Tighten cable clamp at carburetor bracket and check
operation of choke valve to ensure full closed and
wide open positions.
SPRING
CHEVROLET TRUCK SERVICE MANUAL