guest18,
my old carb engine also like that.. but its between 700rpm and 1300rpm lar, with aircond off. if aircond on then rpm drop 200.
i asked around already, they told me its because when the engine is damn hot, the engine oil get thinner, thus the engine somehow go faster. *scratch head* i also dunno why and never knew got this thing, dunno its true or not. but since it didn't give me any problem, even when idle at 500rpm the car won't shake, so i didn't care.
now i'm using efi, i just twist the screw on my air flow meter can change the idling rpm already, and then there's another screw for me to tune the idling point when i have the aircond switched on.