Why does the lawn mower engine speed run up and down after running out of gas?

There was a guy who invented a .... thing shall I say for this exact example. He designed a ...THING ... that controlled this combustion at the end of a lawn mower cycle and manipulated it to work in a car. The car got 145 miles to the gallon, yes thats right, 145 MILES to the GALLON, he patented it and sold around 26 before a certain oil company got wind of this guys patent. This oil company then went and bought this patent and then tried to get back the 26 units already sold. Some people said no, a class action was made and to my disbelief the people got to keep the units. But yes, the particular design has never been seen again. Go figure.