Jay Maynard writes:

With all that said, a fuel-efficient aircraft engine is a Good Idea, not
because of global warming (boy, have we had to shovel a lot of that out of
our driveway this winter!), but simply because it's less expensive to
operate. In aviation, that's always desirable. It's one reason I'm even
looking at a Rotax-powered aircraft, despite my lingering misgivings about
having an engine my local mechanic can't fix.

So why are the vast majority of small aircraft powered by engines that were
designed during the Second World War at the latest?