Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 44Get this manual
Ignition Diagnostic Procedures TROUBLESHOOTING TIP: With engine running, use inductive timing light to check spark advance of each cylinder as throttle is opened and closedIf timing advances and retards smoothly on each cylinder, ignition system is MOST LIKELY functioning properlyIMPORTANT: If outboard appears to have an ignition system failure, it is recommended that before beginning in-depth troubleshooting: aEnsure that the engine is mechanically sound condition(Fuel System, Cylinder Compression etcbCheck all engine ground leads for loose or corroded connectionsPROBLEMCONDITION No Spark Condition (Both Cylinders)
cDisconnect and reconnect ignition harness connectors to verify proper continuitySUGGESTED TESTING PROCEDURES
NOTE: The following recommended tests and probable causes are not listed in any specific orderThe technician should use this table as guide to help isolate and test the specific problemconditionAlways perform the DVA tests first (if applicable), then perform resistance test to validate suspected component failure