What degree do I set the ignition timing for 1987 Chevrolet S10 2.5L 4cylinder?

You can't simply set a single "degree" for the ignition timing on a 1987 Chevy S10 2.5L 4-cylinder. The correct timing is dependent on engine speed (RPM) and is controlled by the Electronic Control Module (ECM). There's no static timing setting you can adjust with a distributor like on older carbureted engines.

Attempting to adjust the timing yourself on this electronically controlled system is likely to cause damage or prevent the engine from running correctly. The ECM manages the timing based on various sensor inputs to optimize performance and emissions.

If you're having issues with the ignition system, you should:

* Check for diagnostic trouble codes (DTCs). Your S10 likely has a diagnostic port that can be read with a scan tool or code reader to pinpoint the problem.

* Have the ECM checked. A malfunctioning ECM can cause timing problems that can't be fixed by adjusting any timing setting.

* Consult a repair manual. A factory service manual for your specific year and engine will provide detailed information on diagnostics and troubleshooting.

* Take it to a qualified mechanic. This is the safest and most efficient way to resolve ignition issues in a vehicle with electronic ignition control.

Trying to manually adjust the timing will likely result in poor performance, damage to the engine, and a waste of time and effort.