Possible causes
1. Dirty air filter or to much oil in the air filter.
2. Stuck choke
3. Stuck float
4. Tranny fluid seeping past the main seal and getting into the crank case. If there is white smoke coming out of the pipe and it smells like burnt oil then the seal is probably the issue.
Fixes
The first three are pretty self explanatory. Cleaning the air filter and the carb should take care of these.
The fourth is a little bit more involved. If you have never replaced a main seal then get yourself a service manual if you haven't already or take it to the shop.
This, of course, is not a complete list of possible causes.
Scott