If you're sure there's no air leaks, Try adjusting the air/fuel mixture screw on the bottom. See if richening it up (back the screw out) helps. If not, try adjusting the needle's position. At the top of the slide valve, there's supposed to be a gasket between the slide valve's cover and the carburetor body. When removing/reinstalling that rubber gasket sometimes gets out of whack and will allow a small air leak.