The image below is of a "Predator 670" engine with the fan shroud removed. These engines are available in the United States at Harbor Freight. It is a simple air cooled twin cylinder design, commonly used for farm implements and go karts.
With the shroud removed, we can see the blades of the engine. If you look closely, you can see that some of the fan blades are closer together compared to the others. They gradually get closer together, then gradually get farther apart again in 180 degrees. So the blade opposite any given blade has identical spacing.
What purpose does this variation in the spacing of the fan blades serve?
Image credit: CarsandCameras YouTube video