By the letter in the number
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.
Yamaha outboards use the last letter in the model number to determine the year model. This letter is referenced to a model year chart to find the year of manufacture.
1991
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.
1986 im pretty sure
Two items are required to determine the year model of a Mercury outboard, the serial number, and the horsepower rating.
Johnson and Evinrude outboard motors use the engine "model" number to determine the year model.
1990
To determine the age of a Mercury outboard, you will need to know the engine serial number, and the horsepower rating.
The dry weight of a 1991 model, 130 hp Yamaha outboard is 357lbs.
To determine the year model for a Johnson or Evinrude outboard, the model number, rather than the serial number, is needed.
You need model number to determine model year.