Honda Sky 50cc revving too high on idle - how do I fix this?

I have a honda sky (sgx50) and it has recently started revving very high on idle (equivalent to about three quarters full throttle) for about five minutes and then stalling when the engine heats up. After checking the carburetor assembly I could find nothing wrong. I suspect the auto choke as the carburetor seems fine and although the throttle cable has a bit too much play in it, it functions well enough and doesn't explain the high revving from idle. I'm somewhat stumped as it the engine is acting like the throttle is stuck open, but there are no physical symptoms, at least not in the carburetor.

I understand there is a throttle stop screw that might be holding it too far open, but I'm not sure I know which screw this might be.

Would appreciate any input from anyone reading this.

Niall