[JP.]

Well, any carb I've ever seen is made out of aluminum, so rust is not an issue.but ...

..... Carbs are usually made from aluminum so rust is nothing to be worried about. .....

Alloy corrodes heavenly under influence of water. Rust is corrosion so don't be fouled by aluminium don't rust. Its not brown but white rust.

Water is whats causing all the carb issues with those ethanol blended fuels these days. Enthanol is hygroscopic, it attracts water which causes corrosion even in the deep insides of the carb.

This is how it looks like, carburettor from my '74 Capri V6


Better to dip the "fully stripped" carburettor in a bath of paint thinner for 24 hours.