Remove the air cleaner assembly, disconnect and plug the gas and oil lines, loosen the screw connecting the carb to the manifold, and remove the cables by loosening/removing the allen head screw that attaches them and pulling the cables out, and the carb will be able to be pulled from the manifold. (Use care when reinserting the cables, they have slots in them and can only go back in one way. You'll see what I mean when you remove them.) There are 4 phillips head srews that hold the top and bottom half of the carb together; seperate the carb halves, and make sure all jets are clean (use carb cleaner and compressed air). Let the parts (non plastic) soak in cleaner overnight if you can, it helps loosen the gunk. You'll also need to replace the paper gasket between the carb halves upon reassembly.
If your tank has rust issues, you should consider adding a see through plastic fuel filter to your fuel line from the tank to the carb. Hope all of this helps.