Here's my take on float level. I'm sure someone will correct me if I wrong, but it's worked for me for 40+ years. I like to set my float level at factory specs just as a known starting point so I have a known value before I start fooling around with jets for higher altitude applications. In the extremes of float level adjustments, to low of a fuel level will starve the jets and to high will over flow the bowl. Between those limits there is a small level of "adjustment". The higher the fuel level the more head pressure you have on the jets (pressure is dependent on the height of a liquid) and on top of that is air pressure (variable due to altitude) trying to push the fuel up through the jets into the carb throat. When the piston drops and atmospheric pressure pushes air through the carb and Mr. Bernoulli takes over and drops the pressure in the carb throat, the liquid head pressure and air pressure pushing on the fuel can now push fuel into the carb intake flow. I don't know how much difference a mm more or less of fuel level influences rich/lean mixtures (while you are running down a rocky jeep trail bouncing all over the place) has, But I still like knowing I've got my fuel level set in that sweet spot some where in the middle.
Here's a thread on float level symtoms. Unfortunately the link in the referenced post doesn't work anymore, but still interesting and there's lots of discussion on the web on this topic.... http://www.advrider.com/forums/showthread.php?t=110771