The little needle valve that is in the carb can get very crudded up. It only has a little area around it for the gas to go through. If you take the bottom bowl of the carb out, look at the float needle attached to the bowl. It is probably munked up with years of gas going through it. Also, the jet (should be sorta near the center of the carb when looking at it from the bottom) could be clogged completely shut. Use a very thin and stiff wire (like out of a bristle brush) and push it through the hole. There may also be an idle jet beside it. Double check the choke while you have the carb off. Make sure it's working correctly. No chokey ,no gassy, no cranky.
You might spray a little gas (mixed) into the carb just to see it turn over under power (its always grattifying to heer the motor pop over after working on it for a while).
BE VERY careful about the parts in the carb. They are easy to loose. I was cleaning one on a little suzuki and lost the little needle valve in the grass. (walking from the shop to the moped, damn thing jumped out of my hand on its own I think)
The new one was 28.00 from the dealer and took 8 days to come in. (they might have had one in stock and just wanted me to suffer. The suzuki fa50 has the same carb that the current 50cc mini dirt bikes use).
Good luck!
Jamie