THX, tjpoc!
I don't have a lumpy cam, just the ordinary stock one. I just thought of one thing, my secret theorie (hehe):

If you increase the spark gap the spark ignites more fuel in the same time, because there is more fuel between the electrodes. (always assuming your spark is strong enough.) Then the mixture should actually burn quicker and give more power, using the same amount of fuel, so you don't need so much accelerator and use less fuel to get the same power, i.e. with a bigger gap you get better mileage so long as your spark stays strong enough (and the electrode doesn't hit the piston)

And now the question: Is my theorie correct?
I would appreciate all kinds of answers to this question.
And why would different engines require different gaps?