Air Shutter

On the long pipe that the gas/flames come out of, near the open end where it connects to the gas, there is an adjustable opening to allow it to mix with the gas. It's held tight which a Phillips head screw. Loosen the screw and adjust the size of the opening, then tighten the screw again. I agree, the book was quite confusing and the picture helped not at all.

The air shutter on the main burners at right at the front of each cylinder and are adjusted by loosening one screw and turning the shutter to the proper size. In the instructions, it also says to adjust for the rear burner, but the rear burner on my grill was completely non adjustable. After installing the proper gas nozzle, everything works fine.