2001 Mercury Mariner 50-60HP Factory Service Manual, Page 133Get this manual
TIMING, SYNCHRONIZING,ADJUSTING
4Read the vacuum of cylinder #4It's not important to be at any specific vacuum setting5Adjust the carburetor synchronization screws of cylinders #3, #2 and #1 to match the base vacuum of cylinder #4
NOTE: 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