There's no room on top of the cylinder for a spark plug so a spark plug is put on the side. That gives a funky asymmetrical burn so a second plug is put on the other side to give two flame fronts that meet in the center. A good thing for an engine that has to cruise at over 4500 rpm on the freeway.i have a 86' vulcan vn750. My manual says I have twin cylinders. So why do I have 4 spark plugs?