Because engines are run slightly rich to prevent overheating damage. The extra fuel cools things but, of course, has pollution consequences. (Which is why bikes have lean spots around the RPM range where EPA testing is done.) But the increase of fuel to air needed is a few percent, not large factors as HHO proponents imply. Anyway, we can argue until we're blue in the face. Let's see how your tests work out.
Dirk