I have a bear tracker 250 with a rebuilt motor and a new carb. It revs up fine in neutral and in gear with the wheels off the ground but hardly moves at all when there's a load on it. It will lurch forward slightly then bog down.

 

Check the main and pilot jet on the carb. I got a used bear tracker and the previous owner bought a knock off carb with the wrong jets. I got the proper jets and it runs great now.

 

I had one of those and just sold it. From the factory they are jetted quite lean. If you just remove the airbox lid it will allow to much air and it will bog so choke needs to be used. When everything is closed up tight it runs great but any air leak causes a bog. Now....not sure if that is your problem or not, so first question is ...is that the factory carb or an aftermarket carb? Are you running it with the air box closed up tight? Does the carb need rebuilt? If you have a rebuilt engine and the carb is original then perhaps it is time for a rebuild. Just some suggestions...