start at the carb, and look for vacuum lines
pull them off from the nipples at the carb and listen for changes in idle
if there is no change, you either have ported vacuum, or you found the line with the problem in it
it could also be a bad base gasket at the carb itself
you figure that one out by spraying carb cleaner or starter fluid at the base of the carb
if there is no change in idle, all is well
