carter bbd 2 barrel hissing noise

I have a 1970 duster with a slant six and for awhile there was a hissing noise coming from it and now the car wont idle. when I turn the ignition it starts but as soon as I let go it dies instantly, I was playing around with the idle mixture screws when it started making the hissing noise could that be the reason it keeps dying? or could it be something else? any info will help. Thanks!