If the mower starts and runs, then your best bet is Berryman B-12 Chemtool Total Fuel System Clean-Up (part #2616). Run that at 1 oz/gal for one complete tank of fuel in order to thoroughly clean up the carburetor, intake valve(s), and combustion chamber(s). As long as E10 didn’t sit in the carburetor over the winter and physically damage the carburetor, that should smooth out your idle and restore acceleration.