My small Briggs and Stratton engined cement mixer will only run with my hand over the air intake.?

Hi, after a year or so of storage the little engine will only keep running if I keep my hand over the air intake. I've adjusted the air screw both ways but unless I keep my hand firmly over the intake It will stop. I know I'm making a richer mixture by doing this. I was thinking it may be fuel starvation. I've cleaned the jets where the air screw is and checked the diaphragm , still it does it. Can anyone help?