Seems like a carburetor setting to me!!!
but...
1) Check air leaks
To check if there is an airleak, take off the airfilter and start your engine. Rev it up a little and then put your finger on the carburetor inlet and it should die immediately.
2) Check carburator
If the engines rpm stays high after some high speed runs and then comes back to normal again then the LSN is set too rich. When you lean more the LSN then you will notice that the idle will stay high all the time. To lower the idle you need to cut the air more when the carburetor is in the closed position, this can be obtained by unscrewing more the airscrew.
In your case, try first reach the LSN.
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