the xs400 carbs need a real good clean as they are old and finicky buggers.
try unscewing the pilot mix screw 3.5 turns from bottom, this seems to offset the shite liquid which passes for fuel today!
by carb clean I mean stripped down, everything spotless. does look like it has rust in the carbs before the filter fit?, also the float hight needs checking (26mm top of float to under bowl gasket)