Disassemble the carb, clean it & look carefully at the bits. There is a bit of crap somewhere in there that is jamming the works, or has caused a scratch, or gall in the metal. Polish off any abrasions or burrs. Check the spring that stabilizes the throttle barrel -- make sure that it isn't broken or installed off-center (or backward). Check the barrel retaining screw & make sure that the end is not burred & then put it back togegher carefully so that it is not jamming the barrel (most likely scenario). A good idea is to get help from someone who knows their way around a motor -- they're pretty simple & they're easy to fix & set-up.

There is no reason to buy a new carb or motor.