2000-2005 Yamaha F40B Outboard Service Manual, Page 94Get this manual
1CONTROL SYSTEM
SYNCHRONIZING THE CARBURETOR
Do not adjust the carburetor when it is operating properlyExcess adjustment may cause the engine poor performance
1Check: Engine idling speed Unstable -' Adjust2Adjust: Carburetor synchronization Adjustment steps (1) Remove all of the plugs CD(2) Install the adaptors CD and vacuum gauge Q) onto the intake manifold as shown
Vacuum gauge set VU-0803090890-03159