ORIGINAL: 1QwkSport2.5r
Mr. Cox. I didnt read the table properly. When I've measured carbs before, I've measured the dia. of the venturi on top of the carb (above spraybar) and also the bottom (below the spraybar) and also the O.D of the ''neck'' and most often I've found the measurement of the venturi of the carb is always smaller than the bottom. I assumed thats what the two numbers meant. I didnt take into account the size of the ''neck''. I havent had my 2 cups of coffee yet.
The diameter above and below the carburator barrel is a way to play with the mixture at the low end mainly, as I see it. At low throttle settings it affects the mixture in a similar effect as an airbleed hole (and screw) will do. On engines without an airbleed hole (or low speed needle) the upper whole is often larger to reduce the fuel mixture at low revs. Some instead have a notch in the throat or on the barrel to create the same effect, i.e. to close downstream first.
Regarding coffé, I'm on my forth for today but it doesn't help much...