Two things to check right off, if you have no vacuum leaks. First check the accelerator pump. When you twist the throttle there should be some fuel squirting into the carbs venturi. Secondly there is a little silver needle jet that sticks up into the venturi and the needle sticks into it. This piece has to be put in right or you can have the troubles you describe. One end has a radius on the inside and the other is square. The radiused end needs to be up so that the needle goes into that.
A few other possibilities: low float level, torn diaphragm on the slide, sticky slide, or carb still dirty or improperly assembled. Hope this helps.