Mercury Mariner 30/40 4-Stroke Outboard Service Manual 1998, Page 118Get this manual
TIMING, SYNCHRONIZING,ADJUSTING
4Read the vacuum of cylinder #3It's not jmportant to be at any specific vacuum setting
5Adjust the carbu retor synchronization screws of cylinders #2 andto match the base vacuum of cylinder #3NOTE: Keep viewing the tachometer, as the engine RPM may fluctuate during adjustmentsKeep adjusting the idle RPM screw in order to keep the engine speed at 1000 RPM
Example
If the vacuum reading of Cyl#3 (base vacuum) is
30 cmthen adjust carburetor synchronization screws of cylinders #2, and #1 to 30 cm
32cm 30cm
28cm 26cm
Cyl#1 CylCyl