So, we can clear the carburetor of blame and also the camshaft timing.
I suggest the next thing to check, is the free fuel flow into the carburetor. If you have already cleaned out the needle and seat plus the carburetor entry fitting, you just need to pull off the fuel hose at the carburetor and see how long it takes for a cup-full of petrol to run out of the hose. (If you haven't cleaned the entry sections of the carburetor, remove the drain plug from the float bowl and make the same test that way.) Do this with the fuel tank cap installed, in case it is the tank vent.
Also, examine the air filter including the pre-filter to ensure they are not causing restriction.
If those parts pass muster, we will have to start looking for some more obscure issues, such as incorrectly assembled or damaged governor linkage.
