The governor mechanism (springs, levers) is not working. Look behind the air filter (which appears to be loose) and see if any springs are unhooked or anything else that doesn't look right. A knowledgeable mechanic could probably straighten it out in a few minutes. You should set the rear wheel height the same as the front if you ever get it running good enough to use.

Ok, please see attachments. I think the springs are correctly installed?

Also, I'm not sure whether there is a jet / air passage in the hole indicated in the photo (below). I tried blowing air in (seems blocked) and tried probing with a pin, but still not sure if there's an air passage, or if there is one, must be completely blocked. The similar size hole opposite has an air passage, which I cleaned, as well as the main jet at bottom of carb.

And with the engine surging you can see the governor, etc

:

And the governor lever:

What do you think? Thanks