The intake vacuum or low pressure pulses that occur in the intake on our single cylinder engines varies wildly with RPMs and load (bigger smaller props for example). At idle a manifold pressure gauge will show the individual low pressure suction pulses very well. As the RPMs increase the pulses get more weakly visible as the pulses blend together. Under a high load the manifold pressure tends to go up towards atmospheric pressure resulting in little to no fuel draw. Full throttle or WOT has a similar effect in that the low pressure zone or point moves towards the front of the throttle (barrel or plate).
I made a few videos a while back when I was explaining how the pulse pumps work on a 4 stroke engine and how they don't work under load at high speed. But you can see the effect in the videos below and how it would affect the fuel draw at the carb.
For example, here is the vacuum gauge on my scooter (when I was testing it out, with no bleed valve to smooth out the pulses) at idle:
http://s27.photobucket.com/albums/c1...gineIdling.mp4
here is the same engine in the scooter travelling up a hill at 50mph:
http://s27.photobucket.com/albums/c1...uumAt35mph.mp4
