Originally Posted by GearNut
Although that sounds more like an air leak, you can verify or eliminate the slide as the problem by removing the air cleaner and looking down the throat of the carburetor while working the throttle grip. Use a mirror if necessary. See if it is in fact getting held up in the slide bore of the carburetor. Usually if this happens though, the engine will just scream at full throttle or close to it.
This is just step one in the diagnosis. Check it out and get back with us, ok?
When you say air leak one problem comes to mind. I had a hard time fitting the carb to the motor it was very tight. I noticed the carb has slots in it where the clamp goes if the carb didnt seat all the way can this be the problem? (maybe sucking in air)