There are many many possiblities for this to occur, I've listed a few below. I suggest checking in this order:
1. Make sure throttle cable is fully extended, also check throttle control on handle bar and make sure it is fully closed (fully forward).
2. Check the thumbscrew located on top of the carb. It should be all the way down.
3. As misterright1__99 mentioned above check the idle screw. I recommend screwing it in all the way, than backing it out about 4 full rotations. Just make sure it isnt holding that slide up.
4. Worse comes to worse, the slide is actually placed in the cylinder incorrectly. Believe it or not, there is a specific way it fits in. Do a forum search and look for another post (I know I saw one) that goes into detail about this. If this is the case, you will have to take the slide out of the carb cylinder and reposition, than screw everything back together and go through steps 1-4 again.
In reference to #4 above, there is a small pin in the carb body that must align with the slot on the carb barrel. Many people fail to see this tiny pin and it will keep the carb barrel from sliding down all the way.