Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 113Get this manual
TIMING, SYNCHRONIZING,ADJUSTING
spinniing attempt to remove on top of cover when checking ignition timingIgnition timing is not adjustableThe Electronic Control Module unit electronically controls the ignition timing Wh en initially running the outboarduse timing light to verify that the ignition timing falls within the timing windows as described within the following tests If the ignition timing does not stay within the timing windows, replace the ignition EC.M unit and retest(Refer to the Ignition Diagnostic Procedures tests in Section 2A IMPORTANT: When checking the timing ith the engine runningone of the following test procedures must be followedCheck maximum timing per specification hile running the outboard.IN TEST TANK ON DYNAMOMETER ON BOAT SECURED ON TRAILER "Backed in Waler"