take off the filter and see if the flap in the carb is closed , do this with the engine turned off . sounds more like the flap in the carb is open , but i dunno even if the flap in the carb is open just a little too much it can cause it to take off as soon as youve started it , if the carb flap is opened way out and you try to start it IT wont start at all , last case senario check the clutch you might have a broken clutch spring and the clutch is instantly engaging because of it ..HMMM most likely a broken clutch spring for sure .