OK Stuart - I get to be the bad guy.
Float adjustment in any carburetor is critical. And this is one of the tuning "tools" that a professional may use to see what may result from changing a jet without actually changing the jet.
A carburetor provides fuel based on negative pressure. For a better understanding, read some of the better carb design books; but in a nutshell:
Air passes over a "pickup point" somewhere in the venturi area. The air will have a certain velocity or speed. This air may be accelerated by "booster" venturi or not depending on the design. The air passing over the pickup point causes a negative pressure at the pickup point. The velocity of the air will change the negative pressure, thus why choosing a correctly sized carb (if not original) is important.
The air above the fuel in the bowl is open to atmospheric pressure (through the bowl vent). In about 5th grade science we all learned that we do not suck tea from a straw, but that we create a negative pressure in the straw and atmospheric pressure above the tea in the glass pushes the tea into our mouth. A carburetor works the same way. "Bleedover" or that point at which the pressure in the bowl conteracts the negative pressure and fuel begins to flow is effected by the fuel level. The fuel lever, of course, is effected by the float setting.
Typically, 1/16th of an inch fuel level change will effect 1 jet size change in the metering.
Other issues that may effect fuel level are fuel pump pressure and fuel inlet orifice.
Float drop is less critical, but should still be adjusted. Generally float drop should be set such that the float may not quite touch the floor of the bowl if the bowl is dry.
On older carburetors (those with floats made from unobtainium or veryexpensivium) we suggest adjusting the fuel level by changing the thickness of the gasket below the fuel valve seat, rather than bending the float.
Obviously, the engine will run on less or more than the perfect mixture. Float setting has less bearing on power than on fuel economy, and driveability.
Jon.
