Is this the typical rectangeled shape carburetor that come on most twin cyl. engines? If so look for two small towers in the casting on top of carb, each will have a small plug or cap in it, lightly drill a small hole in each plug and pry it out, under these caps are the mixture screws. The factory sets the mixture setting very lean to help pass EPA regulations. Over time it will run even leaner. If everything else is in good running condition you can re-adjust the mixture settings and make these engines run smooth as hell. Remember- screw in - LEANER , screw out- RICHER. Usually does not take much to improve it.
