Sounds to me as if your engine was running lean (needing to run on partial choke & rpm surge are the most common symptoms). In any event, rejetting or drilling out the existing jets slightly can often solve the problem.
Poor performance, surging, engines that will not run with the choke in the full off position, etc., have become very common problems in recent years, as emissions regulations have forced engine maufacturers to run their engines a lean as possible to reduce pollution & meet regulations. In spite of using a fuel conditioner (such as Sta-Bil), the slightest amount of debris or fuel varnish/gum deposits will cause problems.
Sounds simple enough, but here's the caveat...
Be warned, shops that knowingly tamper with emissions related components on a CARB or EPA certified engine (basically all engines built since the mid to late 90's) can be fined up to $25,000.00 by the EPA. Equipment & machine owners can be fined as well (up to $2,500.00), if they knowingly tamper with any component that can affect the emissions on a certified engine. 
