Mercury Mariner 25HP 4-Stroke Outboard Service Manual 1997, Page 58Get this manual
Ignition Diagnostic Procedures TROUBLESHOOTI NG TIPWith engine runninguse inductive tim ing light to check spark advance of each cylinder as th rottle is opened and closedIf timing advances and retards on each cylinder, ignition system is MOST LIKELY functioning properlyIMPORTANT: If outboard appears to have an igni-
tion system failure, it is recommended that before beginning in-depth troubleshooting:
aEnsure that the engine is mechanically sound condition(Fuel SystemCylinder Compression etcbCheck all engin ground leads for loose or corroded connectionscDisconnect and reconnect ignition harness connectors to verify proper conti nuitySUGGESTED TESTING PROCEDURES