This is the 4 stoke Honda engine . it does have a high speed adjustment . its the screw farthest away from the cylinder head

Running lean open up the main jet screw the screw on the carb farthest from the engine.

The carburetor is about 15 bucks. I would try that

I know this is really old but my mantis is doing the same. Did you ever find out what it was to fix it?

Dean do you have any spare swede and cabbage. It's Craig just mines died again with not having greenhouses so done them down home

Have you checked the gearbox for damage or lack of grease? Seems to be a final drive problem as it runs ok out of the soil? 👍