Warm DRIVEABILITY Test 11 - Ignition Timing System Check

Connect a tachometer and timing light. Ground the carburetor switch. Start the engine. Raise engine speed to more than 1100 RPM until engine reaches normal operating temperature. On 2-Bbl. models, disconnect and plug the vacuum hose to the computer.

Engine speed should be 450-650 RPM on 2-Bbl. models and 550-750 RPM on 4-Bbl. models. Adjust idle speed as necessary. Basic timing should be within 2° of specifications. See BASIC TIMING SPECIFICATIONS chart. Adjust timing as necessary.

BASIC TIMING SPECIFICATIONS

Application	Computer No.	Specification
Fed. 2-Bbl.	4289813	7
Calif. 2-Bbl.	4289913	7
Fed. & Calif. 4-Bbl.	4289919	16
"	4289921	16

If basic timing is okay, remove jumper lead from carburetor ground switch. On 4-Bbl. models, disconnect and plug vacuum hose to the computer. Connect a vacuum pump to the computer and apply 16 in. Hg vacuum. Raise engine speed to more than 2000 RPM. Timing should be within 4° of specifications. See SPARK ADVANCE SPECIFICATIONS chart.

SPARK ADVANCE SPECIFICATIONS ⁽¹⁾

Application

Computer No.

Spark Advance

Fed. 2-Bbl.

4289813

Calif. 2-Bbl.

4289913

Fed. & Calif. 4-Bbl.

4289919 & 4289921

⁽¹⁾	Specifications are plus or minus 4°.

If spark advance is not within specifications, replace the computer. If spark advance is okay, apply 16 in. Hg vacuum (2-Bbl. models) and 2 in. Hg vacuum (4-Bbl. models) to the computer. Position fast idle speed screw on the 2nd highest step of the fast idle cam. Tap lightly on the intake manifold near the detonation sensor while watching the timing. Timing should retard as you tap on the intake manifold.
NOTE: The amount of timing retard is directly proportional to the frequency of the tapping. Maximum retard is 16°.
If timing does not retard, check the wiring between the detonation sensor and the computer for open or short circuits. If wiring is okay, replace the detonation sensor and repeat step 4). If timing still does not retard, replace the computer.
If timing retards, return engine to idle speed. Unplug vacuum supply hose to the computer and connect a vacuum gauge to it. Vacuum reading should be manifold vacuum. If vacuum reading is not as specified, check and repair vacuum supply from carburetor. If vacuum reading is okay, proceed to WARM DRIVEABILITY TEST 12 .