[kev38spl]

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.

Correct that alloy does corroded, however that picture looks like the corrosion is due to dissimilar metal contact then from water issues. Just my two cents.