[skidlids]

Also makes a difference wether your running CV Carbs, Slide carbs or fuel injection
Engines are essentially air pumps but to work they need need a air fuel ratio within a certain range. One end of the range will give good economy the other the best power.
So its not only a matter of drawing in two lots of 300cc (probably less as engines aren't 100% efficient, especially older two valve SOHC ones) 5000 times a minute but also getting not only the correct amount of fuel but also getting it to mix with the inrushing air as best as you can.

What I would like to know at exactly what point do the injectors switch on to fit in with the opening cams duration