If you are saying that it starts at full speed? It is almost certainly the carb or linkage.
The throttle plate is there to regulate the air flow. It is mounted to a cross shaft. The shaft is offset for the purpose of letting the throttle plate which is bigger than the bore of the carb (venturi) close against the bore because it can't go past 90 degrees to the bore. Suction on the throttle plate happens on both sides of the shaft, so, since the plate has more area for suction on the appropriate side there is constant force trying to close the throttle.
It is possible you have somehow got the throttle plate mounted on the wrong side of the shaft and the throttle is being sucked open.
Give us some more detailed information, if nothing else.