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For your 1993 GMC Truck C1500 1/2ton Sub 2WD:
Firing Order FIRING ORDER 1-8-4-3-6-5-7-2 --------------- | 2 4 6 8 | FRONT | | | ----- | | | 4 3 || OF | |8 6 | | |1 5|| | | 2 7 || VEHICLE | ----- | | | | 1 3 5 7 | --------------- DISTRIBUTOR ROTATES CLOCKWISE
FIRING ORDER 1-8-4-3-6-5-7-2 --------------- | 2 4 6 8 | FRONT | | | ----- | | | 4 3 || OF | |8 6 | | |1 5|| | | 2 7 || VEHICLE | ----- | | | | 1 3 5 7 | --------------- DISTRIBUTOR ROTATES CLOCKWISE
The timing is adjustable, but if it has not been disturbed it should not have to be adjusted.
Start the engine and allow it to reach operating temperature. Stop the engine and connect the timing light to the No. 1 (left front) spark plug wire, at the plug or at the distributor cap. You can also use the No. 6 wire, if it is more convenient. Numbering is illustrated in this section.
Do not pierce the plug wire insulation with HEI; it will cause a miss. The best method is an inductive pickup timing light.
Clean off the timing marks and mark the pulley or damper notch and timing scale with white chalk.
Disconnect and plug the vacuum line at the distributor on models with a carburetor. This is done to prevent any distributor vacuum advance. On fuel injected models, disengage the timing connector which comes out of the harness conduit next to the distributor, this will put the system in the bypass mode. Check the underhood emission sticker for any other hoses or wires which may need to be disconnected.
Start the engine and adjust the idle speed to that specified on the Underhood Emissions label. With automatic transmission, set the specified idle speed in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw.
The tachometer connects to the TACH terminal on the distributor and to a ground on models with a carburetor. On models with fuel injection, the tachometer connects to the TACH terminal on the ignition coil. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Some tachometers won't work with HEI.
WARNINGNever ground the HEI TACH terminal; serious system damage will result.
Aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. If the pulley or damper notch isn't aligned with the proper timing mark (see the Underhood Emissions label), the timing will have to be adjusted.
TDC or Top Dead Center corresponds to 0°B, or BTDC, or Before Top Dead Center may be shown as BEFORE. A, or ATDC, or After Top Dead Center may be shown as AFTER.
Loosen the distributor base clamp locknut. You can buy trick wrenches which make this task a lot easier.
Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation to retard, and against the direction of rotation to advance.
Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.
Reinstall the distributor vacuum line or the timing connector. Correct the idle speed.
Stop the engine and disconnect the timing light.