16:1 actually, but that was what the factory recommended.
This was in the 1960's, before the specifically formulated two-stroke oils became widely available.
Even then, most of the first ones on the market were intended for water-cooled outboard motors, not air-cooled engines!
That only changed when high-performance 2-stroke motorcycles started coming out of Japan in the mid 60's, and not immediately even then. Even in the 70's, Castrol Super Outboard was the specified oil for Echo engines.
