Originally Posted by AZbiker
Also make sure the boots going from the carb to the airbox are soft rubber--exposure to the elements hardens the material and then you won't get a good seal. Also the manifold from the carb to the motor shouldn't be cracked--that can also let air in.
The reason it runs good on the choke is that you're running lean. Either this is the result of a plugged pilot jet (just buy a new jet when you get the carb apart, only costs $5-7), or air entering the system at another place besides the air filter.
My bet is the the pilot jet. You should be able to pull the carb, clean the carb, replace the pilot jet in a few hours if you are slow, an hour if you have done it before.