I leave it to you to do the obvious things like get the free fuel flow out of the tank up to the correct rate and make sure the needle and seat is clear and not sticking. You can waste a lot of time if you ignore the routine stuff. My B&S repair manual does not refer to your engine specifically. It may have the same carburetor as the 253400 engines, which is a cross-over flow jet with integral fuel pump. The idle and high speed mixture needles stick up vertically from right above the venturi, the high speed one being directly above the center of the float bowl. The correct first guess adjustment for the idle mixture screw on that carburetor is one turn, compared with 1.5 turns for the high speed mixture screw. Make final idle mixture control at 1750 RPM by setting the screw halfway between too rich and too lean. The correct float level is checked by taking off the carburetor, removing the float bowl, turning the carburetor upside down, and observing whether the float sits exactly parallel to the bowl mounting surface. If it's not, don't try to fix it by just pushing on the float. Use some science - carefully bend the float tang with long-nose pliers.
