some thoughts:

1. perform a static governor adjustment.

2. Check valve clearance. If the exhaust valve has inadequate clearance, some air will be drawn into the cylinder during the intake cycle.

3. insure the o ring between the carburetor and intake is present and is being compressed.

4. inspect the gasket on the intake tube to make sure there are no missing sections. it may be a good idea to remove to inspect the lower side for gaps or cracks.

5. there may be some undetected blockage in the main fuel jet. Additionally, there may be fuel varnish remaining in the jet. When cleaning, the carburetor cleaner is the best solvent to remove the varnish. IF there is other residue from the ethanol fuel, carburetor spray will not clean this. If carburetor spray was used and the original Viton seat was not replaced, this seat will swell and lower the fuel level in the float bowl.

6. internal governor problem or linkage catching on something.

Thank you for these excellent suggestions. I will try them.