Usually the serial number is on a tag mounted on the bracket that holds the outboard engine to the boat.
On the Yamaha outboard, the model number is located on the port, (left), transom mounting bracket. The transom bracket is the part of the engine that is bolted to your boat transom.
2005
you can't
1991
Yamaha 8B starting in 677 is a 1979 model. That makes it 30 years old in 2009. Regards Steve
By the letter in the number
I HAVE A OUTBOARD C150ztXRU
Yamaha outboards presently uses the last letter in the model number to determine the year model of the engine. Each letter of the alphabet represents a particular model year run.
235386
216793
According to the Yamaha guide, the outbound serial number is located on the label in the clamp bracket. The serial number is helpful when replacing your motor.