This might be a dumb response but did you hook the rod from the carburetor throttle lever to the off set hole at the top of the governor lever? Is the governor spring hooked to the correct hole in the governor lever? Hole number six will give you a no load RPM of 2150 and a load RPM of 2000. Hole on the bottom of the governor lever is number one. The spring tension is adjusted by means of the adjusting screw on the spring to run at 2150 revolutions per minute at no load.
If everything is hooked back up properly the engine should not have the problem you described.