Since the spring for the Governor was stretched also, check that the governor lever is operating at all. It might be a failed governor assembly is not allowing the engine to increase RPM properly when you increase the throttle setting. Since the carburetor would still be opening, the engine would run poorly and too rich and the spark plug would eventually load up with unburned fuel and foul out.
If moving the governor lever manually seems to help, or the lever will not move at all, then that is likely your problem area.
If the governor seems to respond properly, check that your air filter is clean and installed properly as too little air also means too much fuel in the incoming mixture.
If there is still no improvement you might have a problem with the carburetor itself.
You can see more details, such as those below, in your
or at the
site using the model of your engine, which is 143.006714, but we recommend any carburetor work or repair be done by a professional to assure the unit runs safely to its full potential and still meets all emissions standards.