* Camshaft: Different camshafts have different timing requirements. A stock replacement cam will have different needs than a performance cam.
* Ignition system: The type of distributor and ignition components (points, electronic ignition) will influence timing.
* Fuel: The type of fuel (lead, unleaded, octane rating) affects combustion and thus optimal timing.
* Engine condition: Worn components can necessitate timing adjustments.
To find the correct timing for *your* 1970 250 Chevy engine:
1. Identify your camshaft: The best approach is to find the camshaft specifications (if possible). This might be stamped on the camshaft itself, or found in service manuals or online resources specific to your engine's build.
2. Consult a service manual: A factory service manual for your specific year and model of vehicle is invaluable. It will provide the recommended base timing for your engine configuration.
3. Check for markings: Look for timing marks on the harmonic balancer and distributor. These are crucial for setting the timing accurately.
4. Use a timing light: You'll need a timing light to correctly set the ignition timing. The service manual will show you where to connect the timing light and how to read the timing marks.
5. Fine-tune the timing: Even with a base setting from the manual, you might need to fine-tune the timing based on your engine's performance. This usually involves small adjustments to achieve optimal performance and prevent detonation (pinging).
In short: Don't rely on a single number for timing. Consult your vehicle's service manual and use a timing light for accurate and safe setting. A general range for a stock engine might be around 6-10 degrees BTDC (Before Top Dead Center) but this is only a rough estimate and should not be used without consulting a manual.
