Here's a general procedure. Specifics may vary slightly depending on your exact model and whether it has a computer controlled ignition system (most likely). Consult your vehicle's repair manual for the most accurate instructions. This procedure assumes a basic understanding of automotive mechanics.
1. Locate the Timing Marks:
* On the harmonic balancer (crankshaft pulley): Find the timing marks. They are usually a notch or series of marks. Your repair manual will show you exactly where they are and what they represent (often 0 degrees before top dead center (BTDC)).
* On the timing cover: There's usually a pointer or indicator on the timing cover that aligns with the marks on the harmonic balancer.
2. Access the Distributor:
* The distributor is usually located on the top of the engine, near the firewall. You'll likely need to remove some parts for better access, possibly air cleaner, various hoses, and possibly even the fan shroud.
3. Check for Computer Control (Crucial Step):
* 1985 models likely have Computer-Controlled Ignition (CCI): This means the timing is controlled by the Engine Control Unit (ECU) and isn't directly adjustable with just the distributor. Attempting to adjust it manually could damage the system. If it's CCI, you'll need a scan tool to check for any timing-related codes or issues. The timing may be adjustable using the ECM, but a professional scan tool would be needed to read the current timing.
* If it's a purely mechanical distributor: Proceed to step 4. However, this is unlikely on a 1985 G20.
4. Connect the Timing Light:
* Connect the timing light's inductive clamp around the number 1 spark plug wire (this is crucial for accuracy).
* Connect the timing light's power leads to the battery. Ensure the positive lead is connected to the positive (+) terminal, and the negative lead to the negative (-) terminal.
5. Set the Engine to Idle Speed:
* Start the engine and let it warm up to operating temperature.
* Adjust the idle speed to the manufacturer's specifications (usually found in the repair manual).
6. Aim the Timing Light:
* Aim the timing light at the harmonic balancer to illuminate the timing marks.
7. Check the Timing (If Mechanical System):
* Observe the timing mark illuminated by the timing light. It should align with the 0° BTDC mark (or whatever the specified timing is in your manual for idle). Your repair manual should specify the proper base timing.
8. Adjusting the Timing (If Mechanical System, Unlikely):
* DO NOT attempt this if you have Computer Controlled Ignition! If you have a purely mechanical system (again, unlikely for 1985), loosen the distributor hold-down clamp slightly.
* Rotate the distributor slightly clockwise to advance the timing or counterclockwise to retard the timing.
* Recheck the timing with the timing light.
* Tighten the hold-down clamp when you have achieved the correct timing.
9. Recheck and Test Drive:
* Double-check your work.
* After reconnecting the negative battery terminal, test drive the vehicle to ensure proper operation.
In almost all cases, a 1985 Chevy G20 305 will have computer controlled ignition. Attempting to adjust the timing manually without understanding your vehicle's specific system could lead to serious engine damage. It is highly recommended to consult a repair manual specific to your vehicle or take it to a qualified mechanic for diagnosis and timing adjustment. A scan tool is almost certainly needed for proper diagnosis and adjustment.
