Pull the carb off, remove the pump cover, and using some fine sandpaper on a flat surface (like a pane of glass), sand the mounting flange. I'll bet it's warped. Then use the green diaphragm like Cab advised. Set the lever so that at full throttle there's .050 clearance between the lever and the adjustment screw that pushes on it. Lever should move immediately when throttle is cracked open. Fine-tune adjustment if/as necessary for that.