Hi Michael. The symptoms you describe are common, and nearly always result from lean mixture. With Briggs engines the usual cause of lean mixture is that some gum has built up between the carburetor's needle and jet. I suggest you try turning the mixture adjustment screw anti-clockwise slightly and see if that clears up the problem. You may only need to adjust it about an eighth of a turn.
Note that yours is of course a horizontal crankshaft engine, and the horizontal crankshaft Briggs engines have flange-mount carburetors rather than O-ring seals as the vertical crankshaft ones do. It is not unknown for the carburetor's two attaching bolts to come loose, allowing some air leakage across the flange. You should check that the mounting bolts are done up firmly before you begin the tuning process.
