Mercury Mariner 200, 225 Optimax Outboards Service Manual, 90-855348, Page 64Get this manual
1998 OFI Troubleshooting Guide (continued) Symptom 3Engine idle is rough Cause 3.1 Low air pressure in rail (less than 79 psi while running) 3.2 Fouled spark plug See 1.2 Action
Replace spark plug: -If carbon bridges electrode gap or if it is completely black-If it is not firing and is wet with fuelNote: If spark plug is grey or completely black with aluminum specs, this indicates scuffed pistonRemove flywheel and inspect keyMeasure fuel pressureRemove and inspect diaphragms (a special tool is required for assembly)Check for fuel entering induction manifold or air compressor inletFuel pump diaphragm leakingCheck throttle cablecam roller adjustmentSee 3.2