How does float height affect engine performance?

[Mandres]

In reading back through the archives I see a lot of people recommend adjusting the floats to help fix a lean condition.  Why is this?  I don't see how raising or lowering the level of fuel in the bowls would make any difference at all as long as there is fuel available.  Obviously if the float needles aren't sealing you will have a problem with gas flooding the carb, but why would tiny changes to the float height make any difference to a running engine?

-M

[JamesG]

You would think a few millimeters wouldn't make a difference but it does. You can try it yourself by changing it and testing it if you are curious enough to go thru the hassle.

I think its because of the surface tension effect. If the fuel level in the carbs is low the vacumm from the mains has to suck harder on the fuel and draws up less fuel than if it were compeletly emersed (I know its really the air pressure "pushing" it up). Same goes with the pilots which are even higher out of the fuel level. Plus the more fuel in the float bowls means it can run longer at WOT without worrying about refill not keeping up.

THats what I think from all the carb and tuning stuff I've read and experianced. YMMV

[scratch]

Essentially, you are correct; the lower the fuel level, the leaner; higher=richer.  Mostly affects idle and low rpm's, but can affect the high range as well.