Re-jetting the carburetor is an important step to achieving proper performance. All new engines must pass EPA requirements for sale or use in the United States and are designed to do so. The stock airbox and exhaust are also very restrictive. When you add a header and open air filter the engine moves more air in and out. This also causes the air/fuel mixture to become lean. This is why the engine will typically only run with the choke closed, which richens the mixture.
For the 196cc to 212cc engines, we use a "140" emulsion tube (or e-tube) and a .036 or .037 jet. That is a ballpark figure that works for most 196cc - 212cc OHV engines.
Remember, this is called "tuning" and not "setting"! Your results depend on the engine and the altitude at which it will be running.