Throttle cable pulling (or letting) it closed all the way? Return spring missing, hooked in the wrong place? Just a few places to start.

Might check the static governor setting. Loosen the clamp holding the Governor arm to the Governor shaft where it comes out the side of the engine, push the Governor arm to wide open position, turn the shaft clockwise, same direction, as far as it will go, retighten the clamp.

Walt Conner