Rochester Carburetor - GMC 4000 Series Service Manual

NOTE: Fuel seepage at the bowl to air norn
mating surface is normal. This usually shows up
as a discolored carburetor and is caused by wick-
ing of the gasket in the area. Correction involves
a periodic tightening of the bowl to air horn a s­
sembly screw s.
4. Lack of Power - Lack of engine power that
is isolated to the carburetor can usually be cor­
rected by proper carburetor calibration and ad­
justment. Carburetors should be repaired rather
than replaced when a lack of power complaint is
encountered.

ROCHESTER CARBURETOR

ADJUSTMENTS
Correct adjustment of idle speed, ignition
timing, and fuel mixture are extremely important
to ensure proper operation of the system and for
maximum engine performance. A tachometer is
necessary for making the settings as a specific
engine rpm is necessary to obtain correct adjust­
ments.
Refer to "Engine Tune-Up Specifications" in
GASOLINE ENGINES (SEC. 6A) of this manual for
complete tune-up specifications.
PRELIMINARY CHECKS
1. Thoroughly warm up the engine. If the en­
gine is cold, allow to run for at least 15 minutes.
2. Be sure the carburetor is properly secured
to the intake manifold, which will exclude the pos­
sibility of air leaks.
3. Inspect the manifold heat control valve (if
used) for freedom of action and correct spring
tension.
IDLE AND MIXTURE ADJUSTMENT
Idle and mixture adjustments should be per­
formed with the engine at operating temperature,
choke open, and air cleaner installed.
Air conditioning (when used) must be turned
on during the carburetor adjustments. Also, when
equipped with automatic transm ission, apply park­
ing brake and place transm ission in "DRIVE" and
air conditioning in "ON" while making carburetor
idle adjustment.
1. Check the dwell reading.
NOTE: For all engines used on vehicles cov­
ered by this publication, the dwell reading should
be 31 to 34 degrees.
2. Set ignition timing, as indicated by "Engine
Data Chart," page 335.
3. Set the idle speed adjustment screw to the
proper engine rpm, as indicated by "Engine Data
Chart," page 335.
G M C S E R V I C E M A N U A L
steady idle speed.
obtain a 20 rpm drop (lean roll).
idle rpm, as indicated in "Engine Data Chart," page
335.
IDLE VENT ADJUSTMENT (Fig. 18)
casting and bottom surface of vent valve with plug
gauge. Clearance should be 0.050".
screwdriver clockwise (inward) to decrease clear­
ance and counterclockwise (outward) to increase
clearance.
FLOAT LEVEL ADJUSTMENT (Figs. 17 and 19)
choke lever.
main body.
cam by rotating rod. Remove choke rod from choke
lever. Note position of rod in relation to levers for
ease in reassembly.
screw s. There are three long and three short
screw s.
FUEL SYSTEM— GASOLINE ENGINE
Figure 18— Idle Vent Adjustment
4. Adjust the mixture screw for maximum
5. Adjust the carburetor mixture screw in to
6. Adjust mixture screw out % turn.
7. Check idle speed and re-adjust to the proper
1. Hold throttle lever against idle stop screw.
2. Measure clearance between top of air horn
3. To adjust, turn slotted vent valve head with
1. Remove carburetor air cleaner.
2. Disconnect choke wire from connector on
3. Remove fast idle cam retaining screw from
4. Remove choke rod from slot in fast idle
5. Remove six air horn to float bowl attaching
Sec. 6M
Page 329