As the engine warms and gets fully warmed, make sure the choke plate fully opens. Sounds like it's either that or too much fuel flowing proportionally to the air mix. Might need to get a rebuild kit. When cleaning/rebuilding you might need to take a small wire and run through any orifices which don't receive new parts included in the kit. If you clean any tiny orifices / jets with wire, use a strand or two of stranded copper wire so that no surfaces (brass jets) are marred. A thorough cleaning is imperative. Debris as small as a grain of pepper can totally malfunction a carburetor.