Well, with a carbureted engine that has no mechanism to limit RPM, you're probably not actually producing work at maximum RPM, but rather wasting mixture due to

Well, with a carbureted engine that has no mechanism to limit RPM, you're probably not actually producing work at maximum RPM, but rather wasting mixture due to valve float. In essence, you're not burning that mixture. If you did, the RPM would keep increasing until the entire motor disintegrated. With electronic ignition, you usually have a way to limit max RPM by stopping ignition, so you're not producing work (and heat) either way.