Max, at about age 65, still likes to mow his own lawn with a gasoline push
mower. It's not self-propelled, but it does have an engine. He's has the same
mower for like 20 years, and it's been great. Well, a few years ago, he noticed
that the mower was not getting very good gas mileage. In other words, the mower
was using more gas per lawn cut than it used to. This seemed to be getting
worse. So, for example, if one tank of gas used to get him through two lawn
cuttings, now a whole tank would only get him to mow the lawn one and a half
times. He would have to fill it up in the middle of the second cut. So, he knew
something was wrong. He took it to the neighborhood small engine repair shop
and for about $100, they replaced the spark plug, the spark plug wire. They
gave him a new air filter. And they probably did a few other things like clean
the carburetor. Of course, for $4 more, he could have gotten a whole new lawn
mower. But, anyway, after all the repairs, he notices that the gas mileage is
still lousy. He's bummed out, and he wondered whether there's something else
they should have replaced. Of course, there isn't. Now, I will add here the
engine and lawn mower are in perfect working order, the same as they were
perhaps the day he bought the thing. The lawn isn't thicker than it used to be.
The lawn density is the same as it has always been. And the question is, what's
wrong?