It is a commonly-held belief that two-stroke engines are less fuel-efficient than four-stroke engines, and some example BSFC figures appear to confirm this as well.

But what is it that causes two-strokes to be less fuel-efficient?

I used to believe that it is due to the fact that the intake stroke occurs once per two revolutions in a four-stroke engine, in contrast to the once-per-revolution of a two-stroke engine, but am not so sure now.

This Evinrude article suggests that the difference in fuel efficient is down to the method of fuel delivery, so the differences in fuel economy is due to an unfair comparison of older two-strokes with newer four-strokes.

So what factors would explain the differences in fuel economy between the two types of engines?