1970-86 CARBURETED ENGINES
Refer to Fuel System for the procedure to time diesel engines and to Driveability & Emissions Controls to check and adjust the phase timing of the two sets of points in 1970-73 models.If the underhood emissions decal differs from the information given in this repair guide, always use the information on the decal as this reflects the latest changes made during production.
- Set the dwell of the breaker points to the proper specification, on those trucks so equipped.
- Locate the timing marks on the crankshaft pulley and the front of the engine.
- Clean off the timing marks, so that you can see them.
- Use chalk or white paint to color the mark on the crankshaft pulley and the mark on the scale which will indicate the correct timing when aligned with the notch on the crankshaft pulley.
- Attach a tachometer to the engine.
- Attach a timing light to the engine, according to the manufacturer's instructions. If the timing light has three wires, one, usually green or blue, is attached to the No. 1 spark plug with an adapter. The other wires are connected to the battery. The red wire goes to the positive side of the battery and the black wire is connected to the negative terminal of the battery. If the truck has electronic ignition use an inductive timing light as described above.
- Leave the vacuum line connected to the distributor vacuum diaphragm on models through 1980. On 1982-86 models disconnect the distributor vacuum hose from the vacuum controller and plug the hose. Also on Canadian Z24 engines disconnect the air injection hoses and cap the injection pipes.
- Allow the engine to run at the specified idle speed with the gearshift in Neutral with manual transmission and Drive (D) with automatic transmission.
- Point the timing light at the marks indicated in the chart and illustrations. With the engine at idle, timing should be at the specification given on the Tune-Up Specifications chart in this section.
- If the timing is not at the specification, loosen the pinch bolt (hold-down bolt) at the base of the distributor just enough so that the distributor can be turned. Turn the distributor to advance or retard the timing, as required. Once the proper marks are aligned, timing is correct.
- Stop the engine and tighten the pinch bolt. Start the engine and recheck the timing to assure it did not change while tightening the bolt. Stop the engine; disconnect the tachometer and timing light.
- Tighten the distributor lockbolt and recheck the timing.
1986-88 FUEL INJECTED ENGINES
When checking ignition timing on air conditioner equipped trucks, make sure that the air conditioner is OFF when proceeding with the check. Refer to Idle Speed And Mixture Adjustments.
- Run the engine until it reaches normal operating temperature.
- Open the hood, and race the engine at 2000 rpm for about 2 minutes under no-load (all accessories OFF ).
- Run the engine at idle speed.
- Race the engine two or three times under no-load, then run the engine for one minute at idle.
- Check the idle speed. Adjust the idle speed to specifications by turning the idle speed adjusting screw. Refer to the Tune Up Specifications Chart.
- Connect a timing light according to the light manufacturer's instructions. Adjust the timing by loosening the distributor hold-down bolts and turning the distributor clockwise to advance and counterclockwise to retard.
- Recheck timing after tightening the hold-down bolt to assure it did not change while tightening the bolt.