Well it should lower the fuel consumption if you do everything right. If the ignition timing settings are already perfect there isn't much to be done but that's hardly ever the case.

The timing is usually too late on mopeds, it slows them down and energy is lost. If you carefully adjust the timings for the whole RPM range, the engine will work with less fuel and also feel more responsive. Also, when the timing map is correct, the engine works at it's maximum efficiency in lower and higher rpms. I think that normally there are some sacrifices on lower RPMs when engines are set to perform their best at higher rpms.