Accelerator pump service may be in order if the engine stalls or hesitates accelerating at low speeds. A good tip to remember is that if an engine at operating temperature coughs through the carburetor and the air horn appears sooty, the mixture is probably too lean. An over-rich mixture could cause the engine to lag or hesitate but it normally won't cough.
This can be caused by a warped air horn (bowl cover), which causes an air leak between the fuel bowl and venturi section.
For all drivability complaints, check all vacuum hoses to make sure they are properly connected. A disconnected hose can allow air leaks which give lean mixture as well as problems of improper action of the component operated by the hose. Also ignition contact point gap and ignition timing should be checked on al drivability complaints. Engine sags, die outs and pop back during warm up are usually caused by retarded ignition timing, improper carburetor choke vacuum kick adjustment, cam index setting, fast idle speed, and mixture setting.
Retarded basic ignition timing causes pop back during starting. Adjust to specification on vehicle emission control label +_ 2 1/2 deg for best engine operation.
On engines equipped with solenoid spark advance for starting check to make sure it is operating. To check disconnect leak to solenoid at bullet connector and energize solenoid momentarily with a jumper. The engine will speed up approximately 50 RPM and timing advance 7 1/2 deg if solenoid is operating.
The carburetor choke vacuum kick locates the choke blade in the best position for good engine operation during warm up. If die out occurs during warm up check the vacuum kick adjustment. The following picture and description illustrates and describes a typical procedure.
Connect and auxiliary vacuum source to diaphragm.
While applying light closing pressure on choke lever check blade opening with a drill or gauge as specified. Refer to shop manual for specific carburetor involved.
Note: A tolerance of +- .015? on 6 cyl engines and +- .10 on v-8’s is allowed on vacuum kick and cam index settings to permit adjustments for best engine operation.
Fast idle cam index position is also important for proper engine speed during engine warm-up. The following figure 2 illustrates a typical procedure. With engine warm adjust fast idle speed on second highest step on cam.
Engine idle speed and idle mixture should be adjusted to specification according to the emission control label on car.