Try looking at these sites for some ideas. I built a version of the Pogue carb 20yrs ago for school on a lawn mower engine and it ran for 4 days on less than 12oz of fuel. It was 20yrs ago, I dont remember the exact amount. Granted, it was not loaded down, just idle the whole time, but you get the idea.
http://blog.hasslberger.com/2007/04/pogue_carburetor_gasoline_vapo.html#more , http://freeenergynews.com/Directory/VapSter/index.html , http://peswiki.com/index.php/Directory:Bourke_Engine You can also look at the US patent office website to search for high milage carbs.