Here's a general outline of the process. Specifics may vary slightly depending on the exact model and modifications. Always consult your vehicle's repair manual for the precise specifications and procedures for your specific engine.
Tools You'll Need:
* Timing light: Essential for accurately reading the timing.
* Socket wrench and extensions: To access the distributor.
* Distributor wrench: To adjust the distributor.
* Chilton's or Haynes repair manual: Provides specific specs for your engine. This is crucial!
* Safety glasses and gloves: Protect yourself.
* Rags: To clean up any spills.
Procedure:
1. Consult Your Repair Manual: This is the most important step. The manual will provide the specific timing specifications (degrees before top dead center - BTDC) for your engine at idle. It will also show diagrams and steps specific to your engine.
2. Prepare the Engine:
* Safety First: Disconnect the negative battery cable.
* Warm-up the Engine: Run the engine until it reaches normal operating temperature. This is important for accurate readings.
* Locate Timing Marks: Find the timing marks on the crankshaft damper (harmonic balancer) and the timing chain cover. Your repair manual will show you exactly where these are located.
3. Connect the Timing Light: Clamp the timing light's inductive pickup around a spark plug wire. Connect the timing light to a power source (usually a battery clip).
4. Find Top Dead Center (TDC) on #1 Cylinder:
* Manually Rotate Crankshaft: Use a wrench on the crankshaft bolt to slowly rotate the engine until the #1 cylinder piston is at top dead center. Your repair manual will tell you how to accurately determine TDC. Look for the alignment marks.
* Verify Compression Stroke: Make sure the #1 cylinder is on the compression stroke (both valves closed). This is important. Your repair manual will describe how to verify this.
5. Set the Distributor: With the engine idling, point the timing light at the timing marks on the crankshaft damper. The mark should align with the specified timing mark (e.g., 10 degrees BTDC - this is a typical value, but your engine might need a different specification).
6. Adjust the Timing (If Necessary): If the timing mark is not aligned correctly, carefully loosen the distributor hold-down clamp and rotate the distributor slightly. Use the distributor wrench for this, and only rotate it a small amount at a time.
7. Recheck Timing: After making adjustments, recheck the timing with the timing light. Repeat steps 5 and 6 until the timing is correct.
8. Tighten the Distributor Clamp: Once the timing is set correctly, tighten the distributor hold-down clamp securely.
9. Reconnect Battery Cable: Reconnect the negative battery cable.
10. Test Drive: Take the vehicle for a short test drive to ensure the timing is correct and the engine runs smoothly.
Important Considerations:
* Vacuum Advance: Many distributors have a vacuum advance mechanism. Make sure it's connected and functioning properly. The timing will be different with and without vacuum advance. Your manual will explain this.
* Electronic Ignition: The procedure may differ slightly if your engine has an electronic ignition system.
* Computer-Controlled Engines: Many 1985 vehicles have computer-controlled ignition systems. Adjusting the distributor may not be possible or advisable. You may need a diagnostic scanner to adjust the timing.
Again, this is a general overview. You MUST consult your vehicle's repair manual for the precise timing specifications and procedure for your specific 1985 GMC 350 engine. Improperly setting the timing can cause engine damage. If you are unsure about any step, it's best to seek professional help.
